1 00:00:08,520 --> 00:00:11,680 Speaker 1: Hey, or hey, have you ever seen a glass frog? 2 00:00:12,360 --> 00:00:12,559 Speaker 2: Mmm? 3 00:00:12,920 --> 00:00:15,680 Speaker 3: You mean one of those transparent amphibians or is it 4 00:00:15,760 --> 00:00:17,759 Speaker 3: like a it'll keep saking me ou out of class? 5 00:00:18,160 --> 00:00:21,000 Speaker 1: I mean they're a real live frog. I was reading 6 00:00:21,000 --> 00:00:24,240 Speaker 1: that they are native to Central America, including Panama, where 7 00:00:24,320 --> 00:00:25,400 Speaker 1: you grew up. Yeah. 8 00:00:25,440 --> 00:00:27,680 Speaker 3: I have heard that, but to be honest, I haven't 9 00:00:27,720 --> 00:00:29,960 Speaker 3: seen one in person. You could probably find them in 10 00:00:30,000 --> 00:00:33,040 Speaker 3: the jungle, but it's not like they're jumping around my house. 11 00:00:33,320 --> 00:00:35,159 Speaker 1: Well did you ever wonder what it would be like 12 00:00:35,280 --> 00:00:37,040 Speaker 1: to be transparent yourself? 13 00:00:37,400 --> 00:00:41,040 Speaker 3: Sounds terrible? I guess you want people to see, right. 14 00:00:42,360 --> 00:00:43,720 Speaker 3: Everyone wants to be seen these. 15 00:00:43,680 --> 00:00:45,760 Speaker 1: Days, unless you want to sneak around the house and 16 00:00:45,760 --> 00:00:46,360 Speaker 1: be invisible. 17 00:00:46,440 --> 00:00:48,920 Speaker 3: I guess if you have the option of turning transparent, 18 00:00:49,159 --> 00:00:52,000 Speaker 3: that's cool. Like, I think all kids dream of being 19 00:00:52,040 --> 00:00:52,960 Speaker 3: invisible at some point. 20 00:00:53,200 --> 00:00:55,520 Speaker 1: I always wondered what happens if you're invisible and you 21 00:00:55,560 --> 00:00:58,000 Speaker 1: take a bite of an apple, Like, can everybody see 22 00:00:58,000 --> 00:01:00,080 Speaker 1: that apple work its way through you? H? 23 00:01:00,600 --> 00:01:03,160 Speaker 3: I guess it depends on you, know, like how the 24 00:01:03,200 --> 00:01:07,080 Speaker 3: invisibility works, Like are you a space that is transparent 25 00:01:07,160 --> 00:01:09,400 Speaker 3: or is it just your molecules are transparent? Or you 26 00:01:09,400 --> 00:01:11,320 Speaker 3: can just eat a cookie mid out of glass frogs 27 00:01:11,640 --> 00:01:14,800 Speaker 3: or eat glass. I guess that sounds less tasty. 28 00:01:14,880 --> 00:01:16,160 Speaker 1: I don't think I want to take a bite out 29 00:01:16,160 --> 00:01:16,360 Speaker 1: of that. 30 00:01:16,680 --> 00:01:34,280 Speaker 3: Yeah, I see you, man, I see you. I am Rhem, 31 00:01:34,360 --> 00:01:36,720 Speaker 3: a cartoonist and the creator of PhD comics. 32 00:01:36,800 --> 00:01:39,720 Speaker 1: Hi, I'm Daniel. I'm a particle physicist and a professor. 33 00:01:39,880 --> 00:01:42,440 Speaker 1: You see Irvine, And even though you can only hear 34 00:01:42,520 --> 00:01:44,360 Speaker 1: my voice, I feel seen. 35 00:01:46,200 --> 00:01:49,120 Speaker 3: Well, that's good. Everyone wants to be seen. I think 36 00:01:49,320 --> 00:01:51,440 Speaker 3: mostly we just feel heard in this podcast. 37 00:01:51,520 --> 00:01:54,040 Speaker 1: That's right. We feel heard, not hurt, because we love 38 00:01:54,080 --> 00:01:57,280 Speaker 1: sharing with you our passion and curiosity about the universe. 39 00:01:57,440 --> 00:02:00,120 Speaker 1: And when I hear back from listeners that's something we 40 00:02:00,160 --> 00:02:03,080 Speaker 1: have said has touched on their deep seated need to 41 00:02:03,200 --> 00:02:06,080 Speaker 1: understand the universe. I do feel heard and seen. 42 00:02:06,400 --> 00:02:09,160 Speaker 3: Yeah, because it is a pretty amazing universe with lots 43 00:02:09,200 --> 00:02:11,440 Speaker 3: to see out there and to hear. I guess if 44 00:02:11,440 --> 00:02:14,200 Speaker 3: you have the right kinds of ears. Because sound doesn't 45 00:02:14,200 --> 00:02:15,679 Speaker 3: travel in space, does it? 46 00:02:15,680 --> 00:02:18,240 Speaker 1: It actually does, just very very slowly. 47 00:02:19,040 --> 00:02:21,400 Speaker 3: Why are you kidding or are you serious? 48 00:02:21,600 --> 00:02:21,720 Speaker 4: No? 49 00:02:21,880 --> 00:02:24,800 Speaker 1: Space is not totally empty, and so in principle, there 50 00:02:24,880 --> 00:02:27,880 Speaker 1: are sound waves that do propagate through the solar wind, 51 00:02:28,000 --> 00:02:30,519 Speaker 1: for example. But in practice, if you're in outer space, 52 00:02:30,560 --> 00:02:32,320 Speaker 1: you're going to freeze before you hear anything. 53 00:02:32,560 --> 00:02:34,560 Speaker 3: Also, you would be hearing it in slow motion, I 54 00:02:34,560 --> 00:02:37,799 Speaker 3: guess right, if sound moves slower, Like, don't go out 55 00:02:37,840 --> 00:02:39,799 Speaker 3: in space too late, you're. 56 00:02:39,680 --> 00:02:43,959 Speaker 1: Dead, that's right. You should shout at your friends in space, 57 00:02:44,120 --> 00:02:47,120 Speaker 1: like a year before they go space walking without a 58 00:02:47,160 --> 00:02:48,280 Speaker 1: sud on so they can hear you. 59 00:02:49,000 --> 00:02:53,079 Speaker 3: Yeah, oh, that's not going to help them. Also, how 60 00:02:53,080 --> 00:02:54,520 Speaker 3: do you shout in space? Like you have to take 61 00:02:54,560 --> 00:02:57,239 Speaker 3: off your helmet to shout, and then that's not good. 62 00:02:57,280 --> 00:03:00,800 Speaker 3: Either in space nobody can hear you. Or you shouldn't 63 00:03:00,800 --> 00:03:02,000 Speaker 3: be hurt or you might get hurt. 64 00:03:02,080 --> 00:03:04,799 Speaker 1: But you should turn your eyeballs up to the night 65 00:03:04,840 --> 00:03:07,360 Speaker 1: sky and wonder how everything works. And you should also 66 00:03:07,440 --> 00:03:09,799 Speaker 1: look down on the ground beneath you and see if 67 00:03:09,800 --> 00:03:13,160 Speaker 1: you can puzzle out the mysteries of everyday objects, the 68 00:03:13,240 --> 00:03:15,920 Speaker 1: nature of our universe, how everything works, how it weaves 69 00:03:15,919 --> 00:03:19,040 Speaker 1: itself together to make this incredible cosmos. That's one of 70 00:03:19,040 --> 00:03:21,680 Speaker 1: the deep mysteries that humans want to unravel. And those 71 00:03:21,680 --> 00:03:23,880 Speaker 1: are the topics we like to take a part on 72 00:03:23,919 --> 00:03:24,680 Speaker 1: this podcast. 73 00:03:24,880 --> 00:03:27,119 Speaker 3: Yeah, and It's amazing and lucky that we can see 74 00:03:27,120 --> 00:03:29,720 Speaker 3: so many things out there in the universe and around us, 75 00:03:29,760 --> 00:03:31,440 Speaker 3: so that we know where they are and we can 76 00:03:31,480 --> 00:03:33,880 Speaker 3: also study them and figure out how they work, what 77 00:03:33,919 --> 00:03:36,640 Speaker 3: they're made out of, and what the rules of this 78 00:03:36,840 --> 00:03:40,240 Speaker 3: crazy universe are. But anyways, welcome to our podcast, Daniel 79 00:03:40,280 --> 00:03:44,240 Speaker 3: and Jorge Explain the Universe, a production of iHeartRadio. 80 00:03:43,600 --> 00:03:45,840 Speaker 1: In which we try to do exactly that, take apart 81 00:03:45,880 --> 00:03:48,960 Speaker 1: the whole universe, see how it works, and explain it 82 00:03:49,160 --> 00:03:51,600 Speaker 1: to you. Some of the questions we love to tackle 83 00:03:51,680 --> 00:03:54,440 Speaker 1: involve the tiniest things in the universe, what the rules 84 00:03:54,480 --> 00:03:56,880 Speaker 1: are for how they work, or the biggest things in 85 00:03:56,880 --> 00:04:00,000 Speaker 1: the universe, like the whole universe itself or super massive 86 00:04:00,080 --> 00:04:03,040 Speaker 1: black holes at the hearts of galaxies. But there's also 87 00:04:03,160 --> 00:04:06,640 Speaker 1: a lot of fascinating physics in between. How those tiny 88 00:04:06,680 --> 00:04:09,440 Speaker 1: little objects pull themselves together to behave in weird and 89 00:04:09,520 --> 00:04:12,800 Speaker 1: wonderful ways. The reason ice cream is so tasty, the 90 00:04:12,840 --> 00:04:16,320 Speaker 1: reason metals conduct electricity, the reason your chair holds you 91 00:04:16,400 --> 00:04:19,320 Speaker 1: up is because of zillions of atoms all working together 92 00:04:19,760 --> 00:04:21,440 Speaker 1: to have fascinating phenomena. 93 00:04:21,640 --> 00:04:24,839 Speaker 3: Yeah, and thankfully we can see them all and hear 94 00:04:24,880 --> 00:04:27,080 Speaker 3: them all so we can study them. It'd be kind 95 00:04:27,120 --> 00:04:29,480 Speaker 3: of hard to know and study the universe if everything 96 00:04:29,520 --> 00:04:31,880 Speaker 3: was invisible, right, or if we didn't have eyeballs. 97 00:04:31,880 --> 00:04:34,760 Speaker 1: In years, that's true, and recently we've discovered the amazing 98 00:04:34,800 --> 00:04:38,120 Speaker 1: fact that most of the universe is invisible. The dark 99 00:04:38,160 --> 00:04:40,839 Speaker 1: matter that's out there is most of the stuff in 100 00:04:40,920 --> 00:04:44,000 Speaker 1: the universe, and we didn't even know it existed until recently. 101 00:04:44,120 --> 00:04:47,960 Speaker 1: Because it's invisible to us. Life passes right through it, 102 00:04:48,279 --> 00:04:50,200 Speaker 1: and there's lots of things going on out there in 103 00:04:50,200 --> 00:04:53,279 Speaker 1: the universe that you just cannot see because your eyeballs 104 00:04:53,320 --> 00:04:55,640 Speaker 1: can't pick them up, you can't taste them, you can't 105 00:04:55,640 --> 00:04:59,000 Speaker 1: smell them. So most of the universe is actually invisible. 106 00:04:59,040 --> 00:05:01,719 Speaker 1: Transparency turns out to be the name of the game. 107 00:05:02,000 --> 00:05:04,919 Speaker 3: So the universe got its kid wish of being invisible. 108 00:05:05,000 --> 00:05:07,200 Speaker 3: Is that what you're saying? What's it trying to sneak 109 00:05:07,200 --> 00:05:07,640 Speaker 3: around and do? 110 00:05:07,800 --> 00:05:09,479 Speaker 1: I don't know, but those dark matter kids are probably 111 00:05:09,480 --> 00:05:14,280 Speaker 1: eating dark matter cookies all night long, and. 112 00:05:14,200 --> 00:05:17,200 Speaker 3: They're made out of dark chocolate and taste better too. 113 00:05:17,520 --> 00:05:20,680 Speaker 3: And my new wish used to be a dark matter person. 114 00:05:21,440 --> 00:05:24,320 Speaker 3: They get to eat dark meat and their chicken. 115 00:05:24,240 --> 00:05:26,320 Speaker 1: Well, It also answers that other question. You know what 116 00:05:26,360 --> 00:05:29,160 Speaker 1: happens when a dark matter kid eats a dark matter cookie. 117 00:05:29,279 --> 00:05:31,960 Speaker 1: Of course it goes through them and becomes dark matter 118 00:05:32,000 --> 00:05:32,599 Speaker 1: on the way out. 119 00:05:32,760 --> 00:05:37,840 Speaker 3: Darker matter, you mean, the darkest matter, because a black hole. 120 00:05:38,720 --> 00:05:40,040 Speaker 3: That's where black holes come from. 121 00:05:40,120 --> 00:05:43,040 Speaker 1: That's exactly right. Black holes are dark matter toilets. 122 00:05:43,480 --> 00:05:46,720 Speaker 3: They're are worse through the things that go into the toilet. 123 00:05:47,400 --> 00:05:49,720 Speaker 1: Here we are elevating the discourse of the nature of 124 00:05:49,720 --> 00:05:50,279 Speaker 1: the universe. 125 00:05:50,360 --> 00:05:52,560 Speaker 3: Yeah, I mean, listeners want to be reached into their 126 00:05:52,560 --> 00:05:54,800 Speaker 3: soul and touched right and seen and hurt. 127 00:05:56,040 --> 00:05:58,120 Speaker 1: I'm not sure they want to have this particular taste. 128 00:05:58,200 --> 00:06:02,039 Speaker 3: However, Well, who said anything about tasting. 129 00:06:02,160 --> 00:06:05,000 Speaker 1: We're talking about eating dark matter cookies. You're fantasizing about 130 00:06:05,080 --> 00:06:07,240 Speaker 1: dark matter chocolate over there. It's all about taste. 131 00:06:07,400 --> 00:06:08,800 Speaker 3: That's why we're all about taste here. 132 00:06:08,960 --> 00:06:10,360 Speaker 1: Good taste, bad taste, you. 133 00:06:10,360 --> 00:06:14,359 Speaker 3: Decide, that's right. We're we're scientists. We explore the full 134 00:06:14,440 --> 00:06:17,800 Speaker 3: range of tastes available to the human experience. But it 135 00:06:17,880 --> 00:06:20,359 Speaker 3: is interesting that even the things that are invisible, we 136 00:06:20,440 --> 00:06:23,479 Speaker 3: can still somehow see them through other things. Right, I mean, 137 00:06:23,600 --> 00:06:27,080 Speaker 3: if dark matter, dark energy was completely invisible. We wouldn't 138 00:06:27,120 --> 00:06:29,440 Speaker 3: even know it was there, but somehow it has an 139 00:06:29,480 --> 00:06:31,800 Speaker 3: influence on things that we can see, thankfully, and that's 140 00:06:31,800 --> 00:06:32,599 Speaker 3: how we know they're there. 141 00:06:32,760 --> 00:06:35,880 Speaker 1: Yes, transparency turns out to be quite a subtle issue. 142 00:06:36,120 --> 00:06:38,719 Speaker 1: Some things can be transparent to one kind of light 143 00:06:38,760 --> 00:06:41,040 Speaker 1: but not to another. Some kinds of light can pass 144 00:06:41,080 --> 00:06:44,160 Speaker 1: through some objects but not other objects. As always, when 145 00:06:44,160 --> 00:06:47,000 Speaker 1: you dig into it, you discover there's a fascinating physics 146 00:06:47,000 --> 00:06:49,120 Speaker 1: story that underlines how things work. 147 00:06:49,360 --> 00:06:51,960 Speaker 3: Yep, this is an interesting topic and so today on 148 00:06:52,000 --> 00:07:01,119 Speaker 3: the podcast we'll be tackling the question how does transpy work? 149 00:07:01,400 --> 00:07:04,320 Speaker 1: Some titled how can your kids eat cookies without you knowing? 150 00:07:04,560 --> 00:07:06,359 Speaker 3: Well, I don't think they need to be transparent to 151 00:07:06,400 --> 00:07:10,880 Speaker 3: do that, depending how sneaky they are and how how 152 00:07:10,920 --> 00:07:13,240 Speaker 3: early are you going to sleep or wake up? Now, 153 00:07:13,280 --> 00:07:16,440 Speaker 3: this is an interesting question, Daniel. I imagine it means 154 00:07:16,480 --> 00:07:19,760 Speaker 3: what makes things see through? Not like how do transparencies 155 00:07:19,760 --> 00:07:23,320 Speaker 3: work or how does government transparency work? Because that apparently 156 00:07:23,400 --> 00:07:26,880 Speaker 3: doesn't work that well or sometimes it works too well. 157 00:07:26,920 --> 00:07:30,120 Speaker 1: That's right. We want sunlight on all the operations of 158 00:07:30,240 --> 00:07:33,480 Speaker 1: the universe, but in particular here we're wondering about why 159 00:07:33,600 --> 00:07:36,760 Speaker 1: light can pass through some kinds of things. Why you 160 00:07:36,800 --> 00:07:39,520 Speaker 1: can see through glass but you can't see through stone. 161 00:07:40,120 --> 00:07:43,640 Speaker 1: Why X rays reveal your soft tissues but not your bones. 162 00:07:43,680 --> 00:07:47,240 Speaker 1: What is the underlying physics? What is the microscopic picture 163 00:07:47,360 --> 00:07:48,400 Speaker 1: of transparency. 164 00:07:49,000 --> 00:07:51,640 Speaker 3: We're exploring the full range of transparent behavior. 165 00:07:51,720 --> 00:07:53,840 Speaker 1: We're just trying to be transparent about how the universe works. 166 00:07:53,880 --> 00:07:56,200 Speaker 3: We're trying to be transparent about how the podcast works, 167 00:07:56,320 --> 00:07:59,720 Speaker 3: is what I'm saying. And to be fully transparent, I 168 00:07:59,720 --> 00:08:03,400 Speaker 3: have not I've read the outlete for today's episode. I 169 00:08:03,440 --> 00:08:05,679 Speaker 3: am kind of winging it today, which is totally different 170 00:08:05,720 --> 00:08:06,680 Speaker 3: from other days. 171 00:08:06,640 --> 00:08:10,400 Speaker 1: Right exactly. If you don't notice the difference between this 172 00:08:10,440 --> 00:08:13,000 Speaker 1: episode and any other episode, then folks, you learn something 173 00:08:13,120 --> 00:08:14,520 Speaker 1: about how the sausage is made. 174 00:08:16,000 --> 00:08:19,480 Speaker 3: You learned that Jorges could thinking done the fly and 175 00:08:19,520 --> 00:08:22,320 Speaker 3: reading really quickly. But anyways, as usually, we were wondering 176 00:08:22,360 --> 00:08:24,680 Speaker 3: how many people out there had thought about things that 177 00:08:24,720 --> 00:08:27,960 Speaker 3: are transparent and if they know how transparency works. 178 00:08:28,200 --> 00:08:30,760 Speaker 1: So thanks very much to everybody who answers these questions. 179 00:08:30,800 --> 00:08:33,520 Speaker 1: It gives us a great insight into what people already 180 00:08:33,600 --> 00:08:35,840 Speaker 1: know and don't know. If you'd like to help us 181 00:08:35,840 --> 00:08:38,880 Speaker 1: out for a future episode of the podcast. Please don't 182 00:08:38,880 --> 00:08:42,560 Speaker 1: be shy write to us two questions at Danielandjorge dot com. 183 00:08:42,600 --> 00:08:44,319 Speaker 3: So think about it for a second. Do you know 184 00:08:44,920 --> 00:08:47,680 Speaker 3: how transparency works? Here's what people had to say. 185 00:08:47,880 --> 00:08:53,680 Speaker 4: Transparency works, in my opinion, by not refracting, or the 186 00:08:53,760 --> 00:08:57,640 Speaker 4: medium that the lightest passing through does not refract in 187 00:08:57,679 --> 00:09:01,040 Speaker 4: any way or change the path of the life. And 188 00:09:01,200 --> 00:09:05,120 Speaker 4: that way you don't get any kind of interference and 189 00:09:05,240 --> 00:09:10,079 Speaker 4: you can see through something it is transparent, so perfectly clear. 190 00:09:10,400 --> 00:09:13,839 Speaker 2: I'm pretty sure that transparency works by like the light 191 00:09:13,920 --> 00:09:17,520 Speaker 2: can travel through the material without being absorbed, so maybe 192 00:09:17,520 --> 00:09:20,520 Speaker 2: there's a lot of spaces between it, and so yeah, 193 00:09:20,520 --> 00:09:23,440 Speaker 2: that light just doesn't get absorbed, so it goes through 194 00:09:24,559 --> 00:09:25,440 Speaker 2: and it's merry way. 195 00:09:25,600 --> 00:09:27,880 Speaker 5: At first, I thought it was simple because the material 196 00:09:27,920 --> 00:09:31,040 Speaker 5: density would lead to more or less transparency, like air 197 00:09:31,200 --> 00:09:34,400 Speaker 5: or some gas and low density would be transparent to 198 00:09:34,440 --> 00:09:37,200 Speaker 5: some point because of the particle density. But then again, 199 00:09:37,240 --> 00:09:40,760 Speaker 5: glass is quite a high density, right, and it's still transparent, 200 00:09:41,240 --> 00:09:46,240 Speaker 5: So maybe it's about reflective index. And yeah, like if 201 00:09:46,280 --> 00:09:48,800 Speaker 5: you put glass in a water that comes invisible because 202 00:09:48,800 --> 00:09:51,560 Speaker 5: it's fully transparent because of the reflective indices, I. 203 00:09:51,840 --> 00:09:55,720 Speaker 6: Would guess transparency has to do with interactions between different 204 00:09:56,080 --> 00:09:59,080 Speaker 6: forces and particles and whether they interact or not. If 205 00:09:59,120 --> 00:10:02,280 Speaker 6: they interact, they are not transparent to each other, and 206 00:10:02,320 --> 00:10:04,800 Speaker 6: if they don't interact, they're transparent to each other. 207 00:10:04,960 --> 00:10:08,440 Speaker 1: It's like a real a race. 208 00:10:09,120 --> 00:10:14,440 Speaker 7: So photon gets emitted, it's absorbed by the first atom 209 00:10:14,840 --> 00:10:18,319 Speaker 7: in the glass, for instance, and then it gets retransmitted 210 00:10:18,320 --> 00:10:21,120 Speaker 7: to the next, and so on and so forth until 211 00:10:21,160 --> 00:10:23,960 Speaker 7: it goes over the other side of the of the 212 00:10:23,960 --> 00:10:28,199 Speaker 7: glass or whatever transparent material we are talking about. 213 00:10:28,360 --> 00:10:30,920 Speaker 8: In order to reflect light, the photon would have to 214 00:10:30,960 --> 00:10:35,360 Speaker 8: be absorbed and then re emitted, I think so. I 215 00:10:35,360 --> 00:10:37,800 Speaker 8: guess in order to be transparent, it would have to 216 00:10:37,800 --> 00:10:43,080 Speaker 8: be something that doesn't absorb photons for whatever reason. That'd 217 00:10:43,080 --> 00:10:43,880 Speaker 8: be interesting one. 218 00:10:43,880 --> 00:10:47,000 Speaker 3: That all right, some pretty intricate answers. I feel like 219 00:10:47,040 --> 00:10:50,720 Speaker 3: we had some hardcore physicists here in the pool today. 220 00:10:50,800 --> 00:10:52,960 Speaker 3: Did you ask people on the internet or in your department? 221 00:10:53,160 --> 00:10:56,520 Speaker 1: These are all Internet answers, and they really reflect the 222 00:10:56,559 --> 00:11:00,720 Speaker 1: incredible complexity of light and matter into it in a 223 00:11:00,760 --> 00:11:04,600 Speaker 1: few episodes, what happens when things reflect? What color means. 224 00:11:04,600 --> 00:11:06,680 Speaker 1: But there really is a whole lot going on here. 225 00:11:06,880 --> 00:11:09,679 Speaker 1: Light bouncing off of matter, or passing through matter, or 226 00:11:09,679 --> 00:11:13,920 Speaker 1: refracting through matter is really a very complicated phenomenon, very 227 00:11:13,960 --> 00:11:17,080 Speaker 1: tricky to understand from the microphysics point of view. 228 00:11:17,880 --> 00:11:21,640 Speaker 3: Yes, I like how it reflects how smarter listeners are. 229 00:11:21,720 --> 00:11:24,360 Speaker 1: But so many really interesting answers, a lot of which 230 00:11:24,440 --> 00:11:26,520 Speaker 1: really get at the heart of the question, which is 231 00:11:26,559 --> 00:11:30,640 Speaker 1: that it's about the interactions of the photons with the material. 232 00:11:30,880 --> 00:11:33,960 Speaker 1: There's also some misunderstanding there, like the idea that light 233 00:11:34,240 --> 00:11:36,600 Speaker 1: goes through things if there are spaces for it to 234 00:11:36,600 --> 00:11:38,760 Speaker 1: like wiggle its way through. That's not really the way 235 00:11:38,760 --> 00:11:40,280 Speaker 1: that we think about it, but we'll dig into it 236 00:11:40,320 --> 00:11:40,880 Speaker 1: and explain it. 237 00:11:40,960 --> 00:11:43,840 Speaker 3: I guess it depends, like the difference between transparency and 238 00:11:43,880 --> 00:11:48,040 Speaker 3: air and transparency in glass and transparency through different kinds 239 00:11:48,040 --> 00:11:49,960 Speaker 3: of light, right, it sort of depends. 240 00:11:50,080 --> 00:11:52,960 Speaker 1: There are definitely differences between air and glass, but the 241 00:11:52,960 --> 00:11:56,480 Speaker 1: basic physics of transparency is quite similar in either case. 242 00:11:56,559 --> 00:11:59,280 Speaker 1: Are the photons like avoiding the atoms, It's not like 243 00:11:59,320 --> 00:12:03,000 Speaker 1: they're fine a pathway through. That's maybe the way transparency 244 00:12:03,000 --> 00:12:05,480 Speaker 1: works for like a screen door. You can see through 245 00:12:05,480 --> 00:12:08,320 Speaker 1: a screen because there are literally holes that a photon 246 00:12:08,400 --> 00:12:10,640 Speaker 1: can pass through. But the reason you can see glass 247 00:12:10,760 --> 00:12:13,800 Speaker 1: is not because it's like a microscreen. That glass is 248 00:12:13,840 --> 00:12:16,520 Speaker 1: finding tiny little holes in the glass to wiggle its 249 00:12:16,559 --> 00:12:17,000 Speaker 1: way through. 250 00:12:17,160 --> 00:12:19,240 Speaker 3: I guess we'll dig into that. And so let's jump 251 00:12:19,320 --> 00:12:22,640 Speaker 3: right in, Daniel, what are the basics what makes something transparent? 252 00:12:22,840 --> 00:12:25,720 Speaker 1: So first let's clarify what we mean by transparent. Right, 253 00:12:25,760 --> 00:12:28,800 Speaker 1: by transparent really just mean that you can see through it. 254 00:12:28,800 --> 00:12:31,120 Speaker 1: It means if you shine a light from one side, 255 00:12:31,240 --> 00:12:34,520 Speaker 1: the light comes out the other side with basically no scattering, 256 00:12:34,640 --> 00:12:36,440 Speaker 1: Like the light is still coherent. If you have an 257 00:12:36,480 --> 00:12:38,760 Speaker 1: image of an ice cream cone on one side, you're 258 00:12:38,800 --> 00:12:41,000 Speaker 1: going to see the ice cream cone still on the 259 00:12:41,040 --> 00:12:43,439 Speaker 1: other side. It might be refracted a little bit or 260 00:12:43,480 --> 00:12:47,640 Speaker 1: bent or distorted, but it's transparent if those images are preserved. 261 00:12:47,760 --> 00:12:49,600 Speaker 3: I see. So like, if light can pass through it 262 00:12:49,640 --> 00:12:52,360 Speaker 3: basically is what it means to be transparent, or information 263 00:12:52,559 --> 00:12:56,080 Speaker 3: from light passes through it without any distortions exactly. 264 00:12:56,280 --> 00:12:58,800 Speaker 1: And even glass, which we think of as transparent, does 265 00:12:58,840 --> 00:13:00,400 Speaker 1: distort it a little bit. Like if you stick your 266 00:13:00,400 --> 00:13:03,000 Speaker 1: finger behind a pane of glass and only half of 267 00:13:03,040 --> 00:13:05,440 Speaker 1: your finger is sticking behind it, you'll notice the finger 268 00:13:05,520 --> 00:13:07,800 Speaker 1: no longer looks like a whole finger. It's like broken 269 00:13:07,840 --> 00:13:10,200 Speaker 1: in half because the path of the light through the 270 00:13:10,200 --> 00:13:12,640 Speaker 1: glass changes a little bit relative to the path of 271 00:13:12,679 --> 00:13:14,920 Speaker 1: the light through the air. That's refraction, which is a 272 00:13:14,960 --> 00:13:17,920 Speaker 1: whole other complicated topic that we dug into in another episode. 273 00:13:18,000 --> 00:13:19,920 Speaker 1: But still you can see your finger right the same 274 00:13:19,920 --> 00:13:22,280 Speaker 1: way you can see through water and you can see 275 00:13:22,280 --> 00:13:25,280 Speaker 1: through air. You can identify things. You can recognize things 276 00:13:25,280 --> 00:13:27,359 Speaker 1: even if the path of the light is slightly. 277 00:13:27,040 --> 00:13:29,440 Speaker 3: Changed, all right. So that means that a light can 278 00:13:30,040 --> 00:13:32,720 Speaker 3: or at least information that the light had before it 279 00:13:32,760 --> 00:13:35,640 Speaker 3: went through the material makes it through unscales. That's what 280 00:13:35,679 --> 00:13:36,600 Speaker 3: it means to be transparent. 281 00:13:36,760 --> 00:13:39,360 Speaker 1: Yeah, And the fundamental thing that's happening there relates to 282 00:13:39,920 --> 00:13:45,079 Speaker 1: how photons either interact or don't interact with the electrons 283 00:13:45,160 --> 00:13:47,920 Speaker 1: in that material. And to understand that, we have to 284 00:13:48,000 --> 00:13:51,120 Speaker 1: understand a little bit about the quantum mechanics of those electrons, 285 00:13:51,160 --> 00:13:54,120 Speaker 1: what energy is they're allowed, and how photons decide whether 286 00:13:54,320 --> 00:13:56,520 Speaker 1: or not to interact with those electrons. 287 00:13:56,559 --> 00:13:59,200 Speaker 3: And you were saying, it's not like it's a screen 288 00:13:59,320 --> 00:14:02,520 Speaker 3: door where like some other light is finding a pass 289 00:14:02,520 --> 00:14:05,240 Speaker 3: through it, But that can also happen, Kennet, Like if 290 00:14:05,240 --> 00:14:10,160 Speaker 3: you have something really thin maybe, or something really sparse 291 00:14:10,400 --> 00:14:12,800 Speaker 3: or light like air, there has to be some photons 292 00:14:12,800 --> 00:14:15,560 Speaker 3: that are making it through without interacting with anything. Right. 293 00:14:15,600 --> 00:14:18,360 Speaker 1: Well, most materials, even like a thin sheet of paper, 294 00:14:18,640 --> 00:14:21,720 Speaker 1: are dense enough that photons are not like finding holes 295 00:14:21,800 --> 00:14:25,120 Speaker 1: in them. The reason that light gets through is because 296 00:14:25,120 --> 00:14:28,200 Speaker 1: the photons are ignoring the atoms. It's not that they're 297 00:14:28,240 --> 00:14:31,600 Speaker 1: missing them, it's that they're passing through without interacting. Because 298 00:14:31,600 --> 00:14:34,720 Speaker 1: when a photon passes by an atom, it doesn't always 299 00:14:34,800 --> 00:14:37,080 Speaker 1: interact with it. There are rules about whether or not 300 00:14:37,080 --> 00:14:39,880 Speaker 1: photons can interact with atoms, and in a solid it's 301 00:14:39,880 --> 00:14:42,200 Speaker 1: complicated because you have lots and lots of atoms. But 302 00:14:42,240 --> 00:14:44,080 Speaker 1: the easiest way to understand it is to start with 303 00:14:44,120 --> 00:14:47,080 Speaker 1: an individual atom. Just think about like a single hydrogen 304 00:14:47,080 --> 00:14:49,760 Speaker 1: atom in space and you shoot a photon at it 305 00:14:49,800 --> 00:14:52,040 Speaker 1: is that photon going to get absorbed by the atom 306 00:14:52,360 --> 00:14:54,880 Speaker 1: or is the photon going to ignore the atom? And 307 00:14:54,920 --> 00:14:58,200 Speaker 1: that depends on the energy of the photon, because these 308 00:14:58,240 --> 00:15:01,560 Speaker 1: atoms can only absorb photon of certain energies. 309 00:15:01,720 --> 00:15:03,880 Speaker 3: Well, I guess it also maybe depends on where you 310 00:15:03,920 --> 00:15:06,800 Speaker 3: shoot the photon, right, Like, if I have an hydrogen atom, 311 00:15:06,840 --> 00:15:09,360 Speaker 3: floating out there in the air and I shoot a 312 00:15:09,480 --> 00:15:12,400 Speaker 3: laser beam, you know, a mile to one side of it. 313 00:15:12,400 --> 00:15:14,280 Speaker 3: It's not going to interact with the hydrogen, is it. 314 00:15:14,400 --> 00:15:16,960 Speaker 1: No, that's right. But if you have a huge wall 315 00:15:17,080 --> 00:15:19,480 Speaker 1: of hydrogen atoms and you shoot the laser beam at it, 316 00:15:19,560 --> 00:15:22,640 Speaker 1: then it's not going to be able to avoid the hydrogen, right, 317 00:15:23,000 --> 00:15:24,720 Speaker 1: It's going to have to get through the other side. 318 00:15:24,760 --> 00:15:27,480 Speaker 1: It's going to have to pass through the hydrogen. And 319 00:15:27,520 --> 00:15:30,080 Speaker 1: that's what makes things transparent. I mean, one thing is 320 00:15:30,120 --> 00:15:31,640 Speaker 1: to have holes in it. Sure, where you can see 321 00:15:31,680 --> 00:15:34,200 Speaker 1: through a screen door, even if it's made of metal 322 00:15:34,360 --> 00:15:36,480 Speaker 1: or stone, as long as there are holes in it. 323 00:15:36,800 --> 00:15:39,760 Speaker 1: But I think the physics of transparency is more interesting 324 00:15:39,760 --> 00:15:43,000 Speaker 1: if we think about what happens when light ignores the material, 325 00:15:43,000 --> 00:15:47,000 Speaker 1: if it passes through the material, rather than finding holes 326 00:15:47,080 --> 00:15:48,000 Speaker 1: or ways around it. 327 00:15:48,120 --> 00:15:51,760 Speaker 3: Well, I know it's more interesting to you as a physicist, 328 00:15:52,120 --> 00:15:54,760 Speaker 3: but I guess for me, I'm just curious about which 329 00:15:54,840 --> 00:15:57,040 Speaker 3: has more of an effect to make something transparent, right, 330 00:15:57,080 --> 00:16:00,480 Speaker 3: Because you know, people always talk about like material like 331 00:16:00,480 --> 00:16:02,720 Speaker 3: our skin, our bodies, they're made out of atoms. But 332 00:16:02,760 --> 00:16:04,680 Speaker 3: the atoms are really sort of far apart and the 333 00:16:04,760 --> 00:16:07,040 Speaker 3: nucleus is really far apart from the electrons, So there's 334 00:16:07,080 --> 00:16:10,000 Speaker 3: a lot of sort of like supposedly empty space, even 335 00:16:10,040 --> 00:16:13,960 Speaker 3: within solid matter like ourselves. I'm just wondering, like, you know, 336 00:16:14,160 --> 00:16:16,160 Speaker 3: there's it seems like there's a lot of space for 337 00:16:16,520 --> 00:16:20,040 Speaker 3: a light to squeeze through or to pass by without 338 00:16:20,160 --> 00:16:22,160 Speaker 3: even knowing there are other things there. 339 00:16:22,240 --> 00:16:24,040 Speaker 1: That's an interesting point, and I think a lot of 340 00:16:24,040 --> 00:16:26,440 Speaker 1: people have maybe the wrong mental picture of what an 341 00:16:26,440 --> 00:16:29,000 Speaker 1: atom sort of looks like to a photon. You know, 342 00:16:29,040 --> 00:16:30,520 Speaker 1: even if you have a grid of atoms, like a 343 00:16:30,520 --> 00:16:33,200 Speaker 1: sheet of atoms making them material, you might imagine that 344 00:16:33,200 --> 00:16:35,640 Speaker 1: it's mostly empty space, as you said. And it's true 345 00:16:35,640 --> 00:16:38,920 Speaker 1: that the nucleus is small compared to the distances between 346 00:16:38,960 --> 00:16:41,720 Speaker 1: the atoms, right, So you have this like grid of atoms, 347 00:16:41,720 --> 00:16:44,040 Speaker 1: and there is a lot of space between the nuclei. 348 00:16:44,200 --> 00:16:46,640 Speaker 1: But that space is not empty, right. That space is 349 00:16:46,680 --> 00:16:51,320 Speaker 1: filled with electrons and electronic fields and forces that are 350 00:16:51,320 --> 00:16:54,680 Speaker 1: holding the nucleus and the electrons together. So the electrons 351 00:16:54,680 --> 00:16:57,760 Speaker 1: are in these buzzy clouds all around the nucleus, and 352 00:16:57,800 --> 00:17:00,800 Speaker 1: the electrons weave the atoms together, right, So it's not 353 00:17:00,840 --> 00:17:04,439 Speaker 1: like there's space between the atoms. The atoms are held together, 354 00:17:04,480 --> 00:17:07,280 Speaker 1: they're in these bonds that tie them together into a 355 00:17:07,320 --> 00:17:10,159 Speaker 1: big grid, into a big lattice, and the electrons can 356 00:17:10,200 --> 00:17:13,040 Speaker 1: sometimes slide back and forth and move between the atoms, 357 00:17:13,400 --> 00:17:15,480 Speaker 1: so there isn't a lot of space between the atoms. 358 00:17:15,720 --> 00:17:18,040 Speaker 1: And then inside the atom, all that space you imagine 359 00:17:18,080 --> 00:17:21,480 Speaker 1: might be empty is really filled with electrons. So from 360 00:17:21,480 --> 00:17:23,800 Speaker 1: the point of view of a photon hitting like a 361 00:17:23,840 --> 00:17:27,000 Speaker 1: sheet of material, a sheet of paper or a sheet 362 00:17:27,040 --> 00:17:29,840 Speaker 1: of glass or whatever, it really is hitting a wall 363 00:17:29,960 --> 00:17:33,320 Speaker 1: of electrons that it can't find a way around unless 364 00:17:33,359 --> 00:17:35,240 Speaker 1: you like physically punch holes in it. 365 00:17:35,480 --> 00:17:37,680 Speaker 3: You just made me wonder, like when you have a material, 366 00:17:37,960 --> 00:17:42,959 Speaker 3: how close together are the electrons shells between atoms, Like 367 00:17:43,080 --> 00:17:45,280 Speaker 3: are are they bumping up against each other or is 368 00:17:45,320 --> 00:17:48,240 Speaker 3: there a certain amount of space between. 369 00:17:48,160 --> 00:17:49,639 Speaker 1: It depends a little bit on the kind of atom 370 00:17:49,680 --> 00:17:52,920 Speaker 1: you have. But atoms tend to bond with their outermost electrons, 371 00:17:53,400 --> 00:17:55,760 Speaker 1: and we'll dig into that if you let me talk 372 00:17:55,800 --> 00:17:59,119 Speaker 1: about how these electrons move between the atoms. But you 373 00:17:59,160 --> 00:18:02,720 Speaker 1: really are tying the outer levels of the electron orbitals 374 00:18:02,760 --> 00:18:06,479 Speaker 1: together and then informing these common electron energy levels. So 375 00:18:06,520 --> 00:18:09,880 Speaker 1: these atoms are sharing electrons that really are woven together. 376 00:18:10,160 --> 00:18:11,879 Speaker 3: I see. I think what you're saying is that to 377 00:18:11,920 --> 00:18:15,240 Speaker 3: a photon a grid of electrons, it's like a solid 378 00:18:15,280 --> 00:18:18,840 Speaker 3: wall almost because it's full of electron fields, and photons 379 00:18:19,200 --> 00:18:22,200 Speaker 3: interact with electrons. But like, if you were something else 380 00:18:22,240 --> 00:18:24,359 Speaker 3: that was not an electron, you would maybe see a 381 00:18:24,359 --> 00:18:26,480 Speaker 3: lot of empty space. But because you are a photon, 382 00:18:26,560 --> 00:18:30,720 Speaker 3: then you're walking into a solid wall of electron fuzziness. 383 00:18:30,800 --> 00:18:34,080 Speaker 1: Yeah, if you're a neutrino, for example, and you don't 384 00:18:34,080 --> 00:18:37,680 Speaker 1: feel electric charge, then the fact that there are electromagnetic 385 00:18:37,720 --> 00:18:40,600 Speaker 1: fields all through these materials is relevant to you, and 386 00:18:40,680 --> 00:18:44,000 Speaker 1: you pass right through it. So like even a block 387 00:18:44,119 --> 00:18:47,960 Speaker 1: of lead is basically transparent to a neutrino because it 388 00:18:47,960 --> 00:18:51,480 Speaker 1: doesn't interact with the stuff. So transparency comes down to 389 00:18:51,520 --> 00:18:54,600 Speaker 1: whether you interact with the material there, not really whether 390 00:18:54,640 --> 00:18:57,480 Speaker 1: there are holes there. So photons they can either pass 391 00:18:57,520 --> 00:19:00,359 Speaker 1: through a material or they can interact with it, and 392 00:19:00,400 --> 00:19:04,000 Speaker 1: that depends on the atomic structure and the energy levels 393 00:19:04,000 --> 00:19:06,560 Speaker 1: that the electrons have and whether or not they interact 394 00:19:06,560 --> 00:19:09,080 Speaker 1: with that photon. Even though a photon sees a wall 395 00:19:09,119 --> 00:19:12,000 Speaker 1: of material, a buzzing blob of electrons in front of it, 396 00:19:12,000 --> 00:19:16,119 Speaker 1: it can still sometimes pass right through without interacting, and 397 00:19:16,160 --> 00:19:17,439 Speaker 1: that's what transparency is. 398 00:19:17,640 --> 00:19:19,880 Speaker 3: Yeah, I guess it can be transparent not just to light, 399 00:19:19,920 --> 00:19:22,600 Speaker 3: but to other things. Right, Like, isn't there a famous 400 00:19:22,640 --> 00:19:26,359 Speaker 3: like gold foil experiment that kind of help people figure 401 00:19:26,359 --> 00:19:27,560 Speaker 3: out the structure of the atom. 402 00:19:27,640 --> 00:19:30,679 Speaker 1: Yeah, that's right. Ruththerford shot alpha particles, which are helium 403 00:19:30,800 --> 00:19:33,919 Speaker 1: nuclei and a very thin tissue of gold, and he 404 00:19:33,960 --> 00:19:37,600 Speaker 1: expected it to mostly just pass through, occasionally get distorted 405 00:19:37,600 --> 00:19:40,640 Speaker 1: a little bit by forces. What he found was that occasionally, 406 00:19:40,680 --> 00:19:44,080 Speaker 1: like one in eight thousand times, the alpha particle the 407 00:19:44,119 --> 00:19:46,800 Speaker 1: helium nucleus bounced right back. Was told him that it 408 00:19:46,880 --> 00:19:49,920 Speaker 1: was interacting with something hard at the core. So that's 409 00:19:49,960 --> 00:19:52,159 Speaker 1: what told him that the sheet of gold actually was 410 00:19:52,240 --> 00:19:55,639 Speaker 1: made out of a grid of hard nuclei, that most 411 00:19:55,640 --> 00:19:58,560 Speaker 1: of the gold was transparent to the alpha particle, but 412 00:19:58,600 --> 00:20:00,320 Speaker 1: that occasional little dots of it we're. 413 00:20:00,280 --> 00:20:04,360 Speaker 3: Not because I guess the helium nuclei would only interact 414 00:20:04,359 --> 00:20:07,360 Speaker 3: with the nuclei of the gold. Right to the helium particle. 415 00:20:07,480 --> 00:20:10,159 Speaker 3: It did look like a screen because there is a 416 00:20:10,160 --> 00:20:13,480 Speaker 3: lot of empty space to a helium atom in between 417 00:20:14,400 --> 00:20:15,560 Speaker 3: the gold atoms. 418 00:20:15,280 --> 00:20:17,240 Speaker 1: Yeah, that's mostly true. The nuance there is that the 419 00:20:17,280 --> 00:20:20,199 Speaker 1: helium does interact with the electrons. It's just that the 420 00:20:20,240 --> 00:20:23,199 Speaker 1: electrons don't have the mass or the kinetic energy to 421 00:20:23,240 --> 00:20:26,000 Speaker 1: bounce it back. They can only slightly change its direction. 422 00:20:26,560 --> 00:20:28,480 Speaker 1: So if the helium goes through the gold and only 423 00:20:28,520 --> 00:20:30,720 Speaker 1: interacts with the electrons, it gets a little bit of 424 00:20:30,800 --> 00:20:33,240 Speaker 1: change of direction, whereas if it hits one of the nuclei, 425 00:20:33,440 --> 00:20:36,080 Speaker 1: those have the mass like push it all the way back. 426 00:20:36,119 --> 00:20:38,760 Speaker 1: So the sort of two different kinds of interaction the 427 00:20:38,760 --> 00:20:41,359 Speaker 1: helium can do, one where it punches through and the 428 00:20:41,400 --> 00:20:42,800 Speaker 1: other one where it bounces back. 429 00:20:42,840 --> 00:20:47,320 Speaker 3: All right, So you're saying, let's maybe focus on light transparency. 430 00:20:47,760 --> 00:20:50,439 Speaker 3: And we know that light interacts with electrons, and so 431 00:20:51,240 --> 00:20:54,399 Speaker 3: material stuff to a photon looks pretty dense and so 432 00:20:54,520 --> 00:20:57,400 Speaker 3: you can't sort of just go through it without possibly 433 00:20:57,400 --> 00:21:00,800 Speaker 3: interacting with things. But you're saying, maybe the key transparency 434 00:21:00,920 --> 00:21:03,960 Speaker 3: is it sometimes like doesn't interact with the thing that's there. 435 00:21:04,119 --> 00:21:06,520 Speaker 1: That's right. Even if you have a bunch of electrons, 436 00:21:06,600 --> 00:21:09,880 Speaker 1: photons aren't necessarily able to interact with them. The photon 437 00:21:09,920 --> 00:21:12,439 Speaker 1: has to have the right amount of energy so the 438 00:21:12,480 --> 00:21:15,560 Speaker 1: electron can accept it. Can absorb it. If photon has 439 00:21:15,560 --> 00:21:20,120 Speaker 1: the wrong energy, it'll pass right through without interacting without electron. 440 00:21:20,240 --> 00:21:22,520 Speaker 1: And that all comes down to how the electrons are 441 00:21:22,560 --> 00:21:25,800 Speaker 1: confined in the material. I mean a random free electron 442 00:21:25,840 --> 00:21:27,960 Speaker 1: in space. So you just have like an electron flying 443 00:21:27,960 --> 00:21:30,399 Speaker 1: through the universe and a photon hits it. It's going 444 00:21:30,440 --> 00:21:33,760 Speaker 1: to absorb that photon no problem, because electrons flying through 445 00:21:33,760 --> 00:21:36,600 Speaker 1: space can have any energy, there's no restriction, can have 446 00:21:36,880 --> 00:21:40,440 Speaker 1: any arbitrary amount of energy, so it will almost always 447 00:21:40,480 --> 00:21:43,840 Speaker 1: absorb that photon. But an electron around an atom has 448 00:21:43,880 --> 00:21:46,600 Speaker 1: different rules. Because of the quantum mechanics. It can only 449 00:21:46,640 --> 00:21:49,560 Speaker 1: exist on like a ladder of energy levels, so it 450 00:21:49,560 --> 00:21:52,720 Speaker 1: can only absorb photons that move it up one or 451 00:21:52,760 --> 00:21:55,560 Speaker 1: two or ten steps on that ladder. It can't move 452 00:21:55,640 --> 00:21:57,600 Speaker 1: up like one and a half steps or two point 453 00:21:57,680 --> 00:22:00,480 Speaker 1: seven steps, so that limits the kinds of poltons that 454 00:22:00,520 --> 00:22:01,800 Speaker 1: an electron can absorb. 455 00:22:02,800 --> 00:22:06,200 Speaker 3: Interesting, it's like the electrons are stuck and they don't 456 00:22:06,200 --> 00:22:09,000 Speaker 3: want to move from where they are, And so let's 457 00:22:09,040 --> 00:22:12,000 Speaker 3: get a little bit deeper into that, and then also 458 00:22:12,160 --> 00:22:14,680 Speaker 3: talk about what happens when something is transparent. So we'll 459 00:22:14,680 --> 00:22:17,000 Speaker 3: talk about that, but first let's take a quick break. 460 00:22:29,520 --> 00:22:32,639 Speaker 3: All right, we're talking about transparency here today and what 461 00:22:32,760 --> 00:22:36,120 Speaker 3: makes things see through. Daniel, you mentioned that it's kind 462 00:22:36,119 --> 00:22:40,240 Speaker 3: of about how light can go into material, see the material, 463 00:22:40,320 --> 00:22:42,359 Speaker 3: be near the material, but not interact with it. And 464 00:22:42,400 --> 00:22:45,440 Speaker 3: you say it has something to do with the energy 465 00:22:45,520 --> 00:22:48,560 Speaker 3: levels of the electron, because you said that an electron 466 00:22:48,600 --> 00:22:51,480 Speaker 3: floating out in space will always absorb an electron no 467 00:22:51,520 --> 00:22:54,240 Speaker 3: matter what does it have to be like flying near it, 468 00:22:54,280 --> 00:22:56,600 Speaker 3: does it have to hit the electron right in the middle, 469 00:22:56,920 --> 00:22:59,680 Speaker 3: or does it interact when it's flying nearby? How does 470 00:22:59,680 --> 00:23:01,680 Speaker 3: that free space case work? 471 00:23:02,000 --> 00:23:04,520 Speaker 1: If you zap an electron with a photon and that 472 00:23:04,680 --> 00:23:07,359 Speaker 1: electron is out in free space, it means that there 473 00:23:07,400 --> 00:23:10,560 Speaker 1: are no rules that govern the energy that the electrons 474 00:23:10,560 --> 00:23:13,359 Speaker 1: can have. I think something that's really cool and not 475 00:23:13,600 --> 00:23:18,000 Speaker 1: like widely enough understood, is where quantization comes from, Like 476 00:23:18,440 --> 00:23:22,960 Speaker 1: why electrons in materials have energy levels. Where these quantum 477 00:23:23,080 --> 00:23:27,000 Speaker 1: energy levels come from? And it really comes from boundary conditions. 478 00:23:27,000 --> 00:23:30,199 Speaker 1: It comes from forcing the electron to live within a 479 00:23:30,240 --> 00:23:32,919 Speaker 1: certain location, like putting it in a box, so that 480 00:23:33,040 --> 00:23:35,760 Speaker 1: electron out in empty space, it can be here, it 481 00:23:35,800 --> 00:23:37,879 Speaker 1: can be there, it can have any location. It can 482 00:23:37,960 --> 00:23:40,680 Speaker 1: have any momentum, and so it's free to absorb a 483 00:23:40,720 --> 00:23:44,120 Speaker 1: photon of any energy. There's a probability for an electron 484 00:23:44,160 --> 00:23:46,280 Speaker 1: and a photon to interact. There's still a chance, of course, 485 00:23:46,320 --> 00:23:48,639 Speaker 1: that a photon will not interact with an electron. It 486 00:23:48,680 --> 00:23:52,640 Speaker 1: depends on the strength of the force essentially that controls 487 00:23:52,640 --> 00:23:55,439 Speaker 1: like the probability for these things to happen. But for 488 00:23:55,520 --> 00:23:58,600 Speaker 1: today's conversation, we can imagine that it basically just always happens. 489 00:23:58,880 --> 00:24:01,679 Speaker 1: You zap an electron with a photon. It doesn't matter 490 00:24:02,080 --> 00:24:05,080 Speaker 1: what the energy of that photon is. The electron can 491 00:24:05,119 --> 00:24:07,520 Speaker 1: accept it because it could have a higher momentum of 492 00:24:07,600 --> 00:24:11,359 Speaker 1: any value. That's not true for electrons around an atom. 493 00:24:11,440 --> 00:24:14,680 Speaker 1: It can only have energies of certain values because it's 494 00:24:14,680 --> 00:24:17,240 Speaker 1: confined into the box of the atom. 495 00:24:17,280 --> 00:24:20,280 Speaker 3: Now, within that free space electron, when that light hits it, 496 00:24:20,320 --> 00:24:22,720 Speaker 3: what happens to that electron It gets faster or it 497 00:24:22,760 --> 00:24:25,320 Speaker 3: gets hotter, and does it start spinning faster? What happens 498 00:24:25,320 --> 00:24:27,520 Speaker 3: when you use zap an electron in space would. 499 00:24:27,400 --> 00:24:30,840 Speaker 1: Like, well, it absorbs the momentum of that photon because 500 00:24:30,840 --> 00:24:33,720 Speaker 1: of conservation momentum, and now it carries that momentum as well. 501 00:24:33,840 --> 00:24:35,960 Speaker 1: And so if the photon was moving in the same 502 00:24:36,000 --> 00:24:38,480 Speaker 1: direction as the electron, then it gives it a zip. 503 00:24:38,680 --> 00:24:40,879 Speaker 1: It's going faster. If the photon was moving in the 504 00:24:40,880 --> 00:24:43,920 Speaker 1: opposite direction, the electron hits the photon sort of head on, 505 00:24:44,320 --> 00:24:47,240 Speaker 1: then it gets slowed down. Right. We talked about like 506 00:24:47,359 --> 00:24:50,560 Speaker 1: laser cooling once on the podcast. You can use lasers 507 00:24:50,560 --> 00:24:54,000 Speaker 1: to slow things down. Also if you zap things in 508 00:24:54,040 --> 00:24:54,719 Speaker 1: the right direction. 509 00:24:55,080 --> 00:24:57,600 Speaker 3: Yeah, that was pretty cool. And so it's sort of 510 00:24:57,640 --> 00:24:59,159 Speaker 3: like a billier ball. I guess, like if you have 511 00:24:59,160 --> 00:25:01,560 Speaker 3: an electron other and you hit it with a photon, 512 00:25:01,640 --> 00:25:04,000 Speaker 3: it basically happens like it does when you hit a 513 00:25:04,000 --> 00:25:05,560 Speaker 3: billier ball, right with the white ball. 514 00:25:05,720 --> 00:25:08,520 Speaker 1: Yeah, the quantum mechanics comes in with the probability there's 515 00:25:08,520 --> 00:25:10,960 Speaker 1: like a chance that the interaction will happen, a chance 516 00:25:11,240 --> 00:25:13,399 Speaker 1: that it won't. But for today's conversation, we can think 517 00:25:13,440 --> 00:25:14,879 Speaker 1: of it like a billiard ball. You're giving it a 518 00:25:14,880 --> 00:25:17,680 Speaker 1: push and it's absorbing that energy. And because a free 519 00:25:17,720 --> 00:25:19,720 Speaker 1: electron one out in the middle of space can have 520 00:25:19,760 --> 00:25:22,280 Speaker 1: any energy, quantum mechanics is fine with that. Now you 521 00:25:22,320 --> 00:25:24,480 Speaker 1: take that same electron, you say, okay, you're now in 522 00:25:24,640 --> 00:25:27,040 Speaker 1: orbit around a hydrogen atom. Well, you still have to 523 00:25:27,040 --> 00:25:29,040 Speaker 1: obey the rules of quantum mechanics, and in this case, 524 00:25:29,119 --> 00:25:32,840 Speaker 1: quantum mechanics says are there's only certain solutions to the 525 00:25:32,880 --> 00:25:36,840 Speaker 1: math here. Only certain energies of the electron make the 526 00:25:36,920 --> 00:25:39,600 Speaker 1: math work. The wave function of the electron has to 527 00:25:39,640 --> 00:25:43,040 Speaker 1: satisfy some conditions, and that's only true for a certain 528 00:25:43,240 --> 00:25:45,760 Speaker 1: values of the electron energy. So you can't have an 529 00:25:45,800 --> 00:25:49,800 Speaker 1: electron with an arbitrary energy around a proton. There's a 530 00:25:49,880 --> 00:25:52,800 Speaker 1: ladder of values there, and that determines whether the electron 531 00:25:52,840 --> 00:25:55,400 Speaker 1: can absorb the energy of a passing photon. 532 00:25:55,800 --> 00:25:57,520 Speaker 3: I guess it's sort of like, you know, the Earth 533 00:25:57,560 --> 00:25:59,639 Speaker 3: is going around the Sun in an orbit, but the 534 00:25:59,680 --> 00:26:02,320 Speaker 3: Earth is it's not restricted to what that orbit can be. 535 00:26:02,440 --> 00:26:05,280 Speaker 3: Like if a meteor hits Earth within a force, it 536 00:26:05,359 --> 00:26:07,240 Speaker 3: is going to speed us up or slow us down, 537 00:26:07,240 --> 00:26:09,800 Speaker 3: and it's going to change the path of our orbit. 538 00:26:10,400 --> 00:26:12,800 Speaker 3: But you're saying sort of like in an electron around 539 00:26:12,800 --> 00:26:15,200 Speaker 3: an atom, it's not like it can be in any orbit. 540 00:26:15,320 --> 00:26:18,800 Speaker 3: It can only be in like certain slots of that orbit. 541 00:26:18,880 --> 00:26:21,760 Speaker 3: Like it can orbit here or over there, or in 542 00:26:21,800 --> 00:26:23,840 Speaker 3: this circle or in that circle. I know it's not 543 00:26:23,880 --> 00:26:25,600 Speaker 3: really a circle, but it's sort of that. I mean, 544 00:26:25,600 --> 00:26:30,399 Speaker 3: it can only circle around the nuclei or a certain grooves, right. 545 00:26:30,480 --> 00:26:33,480 Speaker 1: Yeah, that's a great contrast because the example of Earth 546 00:26:33,600 --> 00:26:36,480 Speaker 1: is a classical example. There's no quantum mechanics there. We're 547 00:26:36,480 --> 00:26:39,320 Speaker 1: talking about gravity, which is a classical theory, and there's 548 00:26:39,320 --> 00:26:42,840 Speaker 1: an infinite number of possible solutions for an orbit. If 549 00:26:42,880 --> 00:26:44,719 Speaker 1: you pick a radius for the Earth's orbit, I can 550 00:26:44,760 --> 00:26:47,080 Speaker 1: tell you exactly what velocity it has to have in 551 00:26:47,200 --> 00:26:49,960 Speaker 1: order to have that orbit. So there's an infinite number 552 00:26:49,960 --> 00:26:52,240 Speaker 1: of possible orbits there. In the case of the electrons, 553 00:26:52,240 --> 00:26:56,000 Speaker 1: it's really very different mathematics that determines whether the electron 554 00:26:56,080 --> 00:26:58,240 Speaker 1: can be in a particular state or not. You said, 555 00:26:58,240 --> 00:27:01,080 Speaker 1: it's not really in an orbit, it's in a quantum state, 556 00:27:01,119 --> 00:27:03,520 Speaker 1: which means it's satisfying a different equation. In this case, 557 00:27:03,560 --> 00:27:06,600 Speaker 1: it's Schrodinger's equation, which is a quantum equation of the 558 00:27:06,640 --> 00:27:10,080 Speaker 1: wave function, and that wave function has periodicity to it, 559 00:27:10,280 --> 00:27:12,600 Speaker 1: so the wave function basically has to wrap itself around 560 00:27:12,640 --> 00:27:15,000 Speaker 1: the atom in a way that builds upon itself. It 561 00:27:15,000 --> 00:27:17,439 Speaker 1: doesn't cancel itself out, So you can fit in like 562 00:27:17,480 --> 00:27:20,760 Speaker 1: an integer number of half wavelengths of this wave function, 563 00:27:21,160 --> 00:27:23,399 Speaker 1: so that things like support each other. You get like 564 00:27:23,440 --> 00:27:27,080 Speaker 1: a standing wave solution effectively instead of things like canceling 565 00:27:27,080 --> 00:27:29,000 Speaker 1: themselves out. Just the same way that like on a 566 00:27:29,040 --> 00:27:32,160 Speaker 1: guitar string, you can have a certain number of modes 567 00:27:32,359 --> 00:27:35,040 Speaker 1: of a guitar string, right, it can oscillate the whole string, 568 00:27:35,359 --> 00:27:36,720 Speaker 1: or you can have a node in the middle so 569 00:27:36,800 --> 00:27:39,040 Speaker 1: both halves are oscillating, or you can have two nodes, 570 00:27:39,080 --> 00:27:42,199 Speaker 1: or you get like three little oscillating pieces. In the 571 00:27:42,240 --> 00:27:45,120 Speaker 1: same way, the electron has to satisfy a wave equation, 572 00:27:45,560 --> 00:27:48,840 Speaker 1: not a gravitational equation, and that's where the energy levels 573 00:27:48,840 --> 00:27:51,879 Speaker 1: come from comes from confining it to being around the atom, 574 00:27:52,080 --> 00:27:54,159 Speaker 1: which changes the solutions to the equation. 575 00:27:54,960 --> 00:27:57,919 Speaker 3: Okay, so now I have an electron orbiting around a nuclei. 576 00:27:58,119 --> 00:28:01,200 Speaker 3: It's a wave function, it's a quantum object. It's sort 577 00:28:01,200 --> 00:28:03,960 Speaker 3: of like snaps into a certain wave shape around the 578 00:28:04,080 --> 00:28:06,520 Speaker 3: nuclei and you're saying that a photon hits it, and 579 00:28:06,560 --> 00:28:09,160 Speaker 3: the electron is like nope, I like where I am now? 580 00:28:10,080 --> 00:28:12,800 Speaker 3: No thanks, or that's not enough to get me to 581 00:28:12,840 --> 00:28:15,600 Speaker 3: the next step in the ladder. I'm just gonna totally 582 00:28:15,680 --> 00:28:17,000 Speaker 3: ignore you. Is that what's happening. 583 00:28:17,200 --> 00:28:20,119 Speaker 1: That's exactly what's happening. If a photon comes along and 584 00:28:20,160 --> 00:28:22,680 Speaker 1: it has enough energy to bump the electron to the 585 00:28:22,680 --> 00:28:25,560 Speaker 1: next stage, it gets absorbed. If it has too much 586 00:28:25,720 --> 00:28:27,800 Speaker 1: energy to get the electron to the next stage and 587 00:28:27,840 --> 00:28:30,360 Speaker 1: not enough to get it like two steps up, then 588 00:28:30,400 --> 00:28:33,199 Speaker 1: it gets ignored. Right, So it gets absorbed if it 589 00:28:33,240 --> 00:28:36,000 Speaker 1: has the right energy to move the electron up one 590 00:28:36,119 --> 00:28:39,520 Speaker 1: or two or seven some integer number of levels, and 591 00:28:39,560 --> 00:28:42,800 Speaker 1: it gets ignored. If the electron would not have a 592 00:28:42,840 --> 00:28:45,640 Speaker 1: solution anymore if it absorbed this photon, then it just 593 00:28:45,760 --> 00:28:47,000 Speaker 1: doesn't happen. Wait. 594 00:28:47,000 --> 00:28:49,360 Speaker 3: Wait, wait, So, like if an electron is going around 595 00:28:49,360 --> 00:28:52,080 Speaker 3: a nuclei, it's in a cloud, and it gets one 596 00:28:52,120 --> 00:28:55,280 Speaker 3: and a half as much energy from a photon that 597 00:28:55,360 --> 00:28:57,280 Speaker 3: it needs to get to the next level, it's not 598 00:28:57,320 --> 00:29:00,239 Speaker 3: going to take that one and then throw away the remainder. 599 00:29:00,360 --> 00:29:02,160 Speaker 3: It's just totally gonna ignore the whole thing. 600 00:29:02,520 --> 00:29:05,360 Speaker 1: Just totally gonna ignore the whole thing. It can absorb 601 00:29:05,440 --> 00:29:08,240 Speaker 1: something that has two steps and then emit one, right, 602 00:29:08,520 --> 00:29:10,560 Speaker 1: or it can absorb something that has like seven and 603 00:29:10,680 --> 00:29:13,840 Speaker 1: emit four photons, But it has to be on that ladder. 604 00:29:14,320 --> 00:29:16,480 Speaker 3: How exact does it need to be, like exactly to 605 00:29:16,560 --> 00:29:19,680 Speaker 3: the one infinite decimal, you know what I mean? Like, 606 00:29:19,680 --> 00:29:22,600 Speaker 3: where are the chances that the photon will have the 607 00:29:22,720 --> 00:29:25,600 Speaker 3: exact amount of energy needs or does it just need 608 00:29:25,640 --> 00:29:27,200 Speaker 3: to be around the same energy. 609 00:29:27,360 --> 00:29:30,320 Speaker 1: Well, there's always uncertainty in quantum mechanics, right, So every 610 00:29:30,320 --> 00:29:32,440 Speaker 1: photon has an uncertain amount of energy. You can never 611 00:29:32,520 --> 00:29:33,480 Speaker 1: measure it precisely. 612 00:29:33,760 --> 00:29:36,920 Speaker 3: Is that true? Really? I thought they had like specific frequencies. 613 00:29:37,000 --> 00:29:39,440 Speaker 1: Well, a photon is created by a quantum process, which 614 00:29:39,480 --> 00:29:42,080 Speaker 1: usually means that there is some uncertainty there. Right, there's 615 00:29:42,080 --> 00:29:44,120 Speaker 1: always a little bit of fuzz in all of these 616 00:29:44,120 --> 00:29:47,160 Speaker 1: processes which allow these things to overlap. The energy levels 617 00:29:47,160 --> 00:29:50,040 Speaker 1: that we're talking about come from a simplified model of 618 00:29:50,080 --> 00:29:53,040 Speaker 1: the nucleus, right, and in reality these things are a 619 00:29:53,080 --> 00:29:54,040 Speaker 1: little bit fuzzier. 620 00:29:54,440 --> 00:29:54,600 Speaker 7: Right. 621 00:29:54,640 --> 00:29:57,320 Speaker 1: The physics is a little bit more complicated. There's other interaction, 622 00:29:57,440 --> 00:29:59,960 Speaker 1: so there's always a little bit of fuzz on these 623 00:30:00,120 --> 00:30:03,120 Speaker 1: energy levels. And the atom that produced the photon on 624 00:30:03,160 --> 00:30:05,200 Speaker 1: the other side of the universe or whatever, may have 625 00:30:05,240 --> 00:30:07,280 Speaker 1: produced it at a certain energy level or a little 626 00:30:07,280 --> 00:30:09,440 Speaker 1: bit higher, a little bit lower, So there's enough fuzz 627 00:30:09,480 --> 00:30:11,960 Speaker 1: and quantum mechanics to mean that there's a non zero 628 00:30:12,040 --> 00:30:14,880 Speaker 1: probability for the photon to have the right energy to 629 00:30:14,920 --> 00:30:16,240 Speaker 1: be absorbed by the electron. 630 00:30:16,520 --> 00:30:20,040 Speaker 3: Hmmm. I guess when they interact, then the wave functions 631 00:30:20,080 --> 00:30:22,040 Speaker 3: collapse and then you figure out if it has the 632 00:30:22,120 --> 00:30:24,520 Speaker 3: right amount. But it seems very unlikely they would have 633 00:30:24,560 --> 00:30:25,640 Speaker 3: the exact same amount. 634 00:30:25,960 --> 00:30:27,880 Speaker 1: Yeah, that's where the fuzz comes in. So you get 635 00:30:27,880 --> 00:30:29,800 Speaker 1: a little bit of width to these things, so you 636 00:30:29,840 --> 00:30:32,160 Speaker 1: have a probability for them to overlap. It's not like 637 00:30:32,200 --> 00:30:34,520 Speaker 1: you're throwing a dart in an infinitely sized board and 638 00:30:34,560 --> 00:30:36,240 Speaker 1: having to hit exactly the right spot. 639 00:30:36,360 --> 00:30:38,280 Speaker 3: I see. It's like you have a fuzzy dart and 640 00:30:38,360 --> 00:30:40,640 Speaker 3: the target is fuzzy too, and as long as you 641 00:30:40,720 --> 00:30:43,000 Speaker 3: sort of get it in the rhine ballpark, then it's 642 00:30:43,080 --> 00:30:45,720 Speaker 3: going to knock that electron or not. 643 00:30:45,960 --> 00:30:48,840 Speaker 1: Yeah, exactly. And that explains a lot of atomic physics, right. 644 00:30:48,880 --> 00:30:52,520 Speaker 1: That explains why certain gases look certain colors. That explains 645 00:30:52,560 --> 00:30:54,040 Speaker 1: why when you have a fire you might get like 646 00:30:54,120 --> 00:30:56,240 Speaker 1: green or blue flashes in it. Or if you did 647 00:30:56,240 --> 00:30:59,240 Speaker 1: that experiment in high school chemistry where you put copper 648 00:30:59,240 --> 00:31:02,080 Speaker 1: in your bunsen and it glows green, explains a lot 649 00:31:02,120 --> 00:31:05,360 Speaker 1: of atomic physics because different kinds of materials have different 650 00:31:05,480 --> 00:31:08,840 Speaker 1: energy levels, so they glow with different frequency photons and 651 00:31:08,880 --> 00:31:12,320 Speaker 1: they can absorb different frequency photons. And that's really cool 652 00:31:12,320 --> 00:31:15,560 Speaker 1: because it means we can tell what's in distant stars 653 00:31:15,600 --> 00:31:17,800 Speaker 1: because we can look at the spectrum of energy that 654 00:31:17,840 --> 00:31:20,440 Speaker 1: they emit. We can say, oh, look, these things are 655 00:31:20,480 --> 00:31:23,600 Speaker 1: emitting photons from the energy level that only comes from copper, 656 00:31:23,680 --> 00:31:26,360 Speaker 1: so we can tell there's copper in that star. Sometimes 657 00:31:26,440 --> 00:31:28,920 Speaker 1: these things appear as spikes in the spectrum. Sometimes they 658 00:31:28,920 --> 00:31:31,600 Speaker 1: appear as dips in the spectrum because like the atmosphere 659 00:31:31,600 --> 00:31:34,520 Speaker 1: of the star is absorbing those photons. But the point 660 00:31:34,560 --> 00:31:36,680 Speaker 1: is that there are energy levels to the atom, and 661 00:31:36,720 --> 00:31:39,240 Speaker 1: those determine whether the photon can interact with the electrons 662 00:31:39,240 --> 00:31:41,280 Speaker 1: around the atom or whether it gets ignored. 663 00:31:41,960 --> 00:31:44,200 Speaker 3: All right, So then we're talking about transparency. And so 664 00:31:44,280 --> 00:31:47,240 Speaker 3: if I shoot a photon at an atom, it's gonna 665 00:31:47,760 --> 00:31:49,880 Speaker 3: get up to the electron cloud there and it's going 666 00:31:49,920 --> 00:31:52,480 Speaker 3: to be like, no, I'm not the right energy. I'm 667 00:31:52,520 --> 00:31:54,800 Speaker 3: just going to keep going. Or is it the case 668 00:31:54,840 --> 00:31:57,560 Speaker 3: that they do interact? But then the end result is 669 00:31:57,560 --> 00:32:00,840 Speaker 3: the same and it just spits out a photon of 670 00:32:00,840 --> 00:32:02,240 Speaker 3: the same energy in the same direction. 671 00:32:02,440 --> 00:32:04,960 Speaker 1: Now, if they're the wrong energy, they just do not interact. 672 00:32:05,000 --> 00:32:07,120 Speaker 1: If they're the right energy, it gets absorbed and then 673 00:32:07,160 --> 00:32:09,160 Speaker 1: it can get re emitted. And that's a whole complicated 674 00:32:09,200 --> 00:32:12,600 Speaker 1: phenomena about reflection and refraction and all sorts of stuff. 675 00:32:12,680 --> 00:32:15,320 Speaker 1: In this case, for transparency, it's more about whether there's 676 00:32:15,320 --> 00:32:17,479 Speaker 1: an interaction. If it has the wrong energy levels, it 677 00:32:17,600 --> 00:32:20,840 Speaker 1: just doesn't interact. But that's the case for a single atom, 678 00:32:20,880 --> 00:32:23,360 Speaker 1: which is not really what's going on when you're looking 679 00:32:23,400 --> 00:32:25,640 Speaker 1: at light going through glass or when you're wondering why 680 00:32:25,880 --> 00:32:29,800 Speaker 1: light doesn't go through metal. It's much more complicated because 681 00:32:29,840 --> 00:32:32,280 Speaker 1: now you're packing a lot of atoms together, and so 682 00:32:32,320 --> 00:32:35,320 Speaker 1: the rules about what happens to those electrons now change. 683 00:32:35,800 --> 00:32:38,800 Speaker 3: M let's dig into that. What's going on there. 684 00:32:38,880 --> 00:32:41,120 Speaker 1: So remember the picture we were talking about earlier. When 685 00:32:41,160 --> 00:32:43,720 Speaker 1: a photon is approaching like a sheet of metal or 686 00:32:43,720 --> 00:32:46,800 Speaker 1: a sheet of iron or a sheet of marble or something, 687 00:32:46,920 --> 00:32:50,280 Speaker 1: it's facing a whole wall of atoms, not individual atoms. 688 00:32:50,360 --> 00:32:52,840 Speaker 1: We talked about the energy levels of an individual atom. 689 00:32:52,960 --> 00:32:55,479 Speaker 1: But when you bring these things together to make a grid, 690 00:32:55,760 --> 00:32:58,520 Speaker 1: then the atoms bond They're not just like near each other, 691 00:32:58,840 --> 00:33:01,000 Speaker 1: they really are bonded. And if you remember your high 692 00:33:01,000 --> 00:33:04,560 Speaker 1: school chemistry, that means that they are sharing electrons. Sometimes 693 00:33:04,560 --> 00:33:07,120 Speaker 1: the electron will like be around one nucleus, sometimes around 694 00:33:07,120 --> 00:33:10,520 Speaker 1: another nucleus. So from the point of view of the electron, 695 00:33:10,640 --> 00:33:13,719 Speaker 1: what happens is that you no longer really belonging to 696 00:33:13,760 --> 00:33:16,600 Speaker 1: one nucleus. Now you can think of like the whole 697 00:33:16,680 --> 00:33:20,000 Speaker 1: grid of nuclei as having a bunch of energy levels 698 00:33:20,240 --> 00:33:23,160 Speaker 1: for the electrons. Some of the inner electrons are trapped 699 00:33:23,160 --> 00:33:26,600 Speaker 1: around nucleus, but the outer electrons can flow between them, 700 00:33:26,720 --> 00:33:29,640 Speaker 1: and that creates a whole complicated set of energy levels. 701 00:33:29,640 --> 00:33:32,880 Speaker 1: And instead of having these very specific ladders, now you 702 00:33:32,960 --> 00:33:36,280 Speaker 1: have this like spectrum energy levels. The electrons have lots 703 00:33:36,320 --> 00:33:39,200 Speaker 1: more options of the energy levels they can be at. 704 00:33:39,880 --> 00:33:41,719 Speaker 3: I guess I wonder if it's sort of like, you know, 705 00:33:41,800 --> 00:33:44,800 Speaker 3: we're orbiting around the Sun and we're sort of stuck 706 00:33:44,800 --> 00:33:47,880 Speaker 3: in this orbit, but if another solar system came pretty close, 707 00:33:48,280 --> 00:33:51,480 Speaker 3: maybe Jupiter might be like, oh, sometimes it might do 708 00:33:51,560 --> 00:33:54,080 Speaker 3: like a little figure eight and sometimes leave our solar 709 00:33:54,080 --> 00:33:56,240 Speaker 3: system and go take a loop around that other Sun 710 00:33:56,280 --> 00:33:58,280 Speaker 3: and then come back and is that sort of what's 711 00:33:58,280 --> 00:33:59,400 Speaker 3: happening to the electrons. 712 00:33:59,520 --> 00:34:02,400 Speaker 1: Yeah, that's exactly what's happening to the electrons. There's lots 713 00:34:02,440 --> 00:34:04,480 Speaker 1: more options for them. They don't have to just stick 714 00:34:04,480 --> 00:34:07,640 Speaker 1: around one nucleus. They interact with lots of different nuclei. 715 00:34:07,880 --> 00:34:10,839 Speaker 1: So that has the consequence of sort of spreading these 716 00:34:11,000 --> 00:34:14,880 Speaker 1: sharper atomic orbitals and making them even fuzzier. So instead 717 00:34:14,880 --> 00:34:18,160 Speaker 1: of even really thinking about energy levels now, physicists talk 718 00:34:18,239 --> 00:34:22,360 Speaker 1: about the possibilities for electrons in these materials as energy bands, 719 00:34:22,880 --> 00:34:24,880 Speaker 1: and you may have heard of like the valence band 720 00:34:25,000 --> 00:34:27,640 Speaker 1: or the conduction band. These are like a spectra of 721 00:34:27,760 --> 00:34:30,920 Speaker 1: energy levels available to the electron. Instead of being more 722 00:34:31,000 --> 00:34:33,880 Speaker 1: like a ladder, they get blurred together, so there's lots 723 00:34:33,920 --> 00:34:37,320 Speaker 1: of really really fine steps. It's still technically a ladder, 724 00:34:37,440 --> 00:34:39,240 Speaker 1: but there's many many more steps there. 725 00:34:39,360 --> 00:34:41,240 Speaker 3: Yeah, I guess it's sort of like in one atom, 726 00:34:41,320 --> 00:34:45,360 Speaker 3: the electron's stuck in one particular rut or groove or orbit. 727 00:34:45,480 --> 00:34:48,280 Speaker 3: That's one extreme. The electron is a free floating electron 728 00:34:48,320 --> 00:34:51,680 Speaker 3: adem sit in space by itself. When atoms are sort 729 00:34:51,680 --> 00:34:54,719 Speaker 3: of bonded together in the material, you're saying the electrons 730 00:34:54,719 --> 00:34:56,520 Speaker 3: are sort of in the middle, like they're not quite 731 00:34:56,560 --> 00:34:59,480 Speaker 3: stuck to one particular atom, but they're not quite free either, 732 00:35:00,120 --> 00:35:02,560 Speaker 3: and so they have limited options. But they don't have 733 00:35:02,920 --> 00:35:04,840 Speaker 3: just one option, and so there's sort of a range 734 00:35:04,880 --> 00:35:06,080 Speaker 3: of photons they can absorb. 735 00:35:06,160 --> 00:35:08,000 Speaker 1: Yeah, exactly. And it depends a little bit on the 736 00:35:08,040 --> 00:35:10,719 Speaker 1: temperature of the object. If the object is really really 737 00:35:10,800 --> 00:35:13,080 Speaker 1: cold and the electrons don't have a lot of energy, 738 00:35:13,120 --> 00:35:16,240 Speaker 1: then they've all like settled down to their minimum energy 739 00:35:16,680 --> 00:35:20,279 Speaker 1: and mostly they are orbiting an individual nuclei and they're 740 00:35:20,320 --> 00:35:22,520 Speaker 1: mostly stuck, and so the electrons don't flow very much. 741 00:35:22,560 --> 00:35:24,600 Speaker 1: If the thing is hot, then a lot of the 742 00:35:24,640 --> 00:35:27,200 Speaker 1: electrons have more energy. They have enough energy to like 743 00:35:27,239 --> 00:35:30,160 Speaker 1: hop from nuclei to nuclei, and so they can flow 744 00:35:30,200 --> 00:35:31,000 Speaker 1: a little bit better. 745 00:35:31,080 --> 00:35:33,200 Speaker 3: Wait, what so that if I heat something up or 746 00:35:33,239 --> 00:35:36,080 Speaker 3: cool it down, I can make it go transparent or 747 00:35:36,120 --> 00:35:36,800 Speaker 3: not transparent. 748 00:35:36,920 --> 00:35:39,040 Speaker 1: No, by heating it up, you're not changing the energy 749 00:35:39,120 --> 00:35:42,560 Speaker 1: levels that are available. You're just changing where the electrons are. 750 00:35:42,920 --> 00:35:45,080 Speaker 1: Like instead of all being in the lowest energy levels, 751 00:35:45,160 --> 00:35:47,480 Speaker 1: now they're in higher energy levels. I'm just talking about 752 00:35:47,520 --> 00:35:49,960 Speaker 1: which energy levels are filled up. In some cases, the 753 00:35:50,000 --> 00:35:52,479 Speaker 1: electrons are sort of stuck when this stuff is cold, 754 00:35:52,520 --> 00:35:54,759 Speaker 1: the electrons fill the lower energy levels and they're more 755 00:35:54,800 --> 00:35:57,120 Speaker 1: stuck to the nuclei, and when the object is warmer, 756 00:35:57,200 --> 00:35:59,120 Speaker 1: they sort of jump out of those and they're freer 757 00:35:59,160 --> 00:36:01,640 Speaker 1: to move around from nucleus to nucleus. 758 00:36:01,920 --> 00:36:05,600 Speaker 3: Cool. Well, let's get a little bit deeper into the 759 00:36:05,760 --> 00:36:09,319 Speaker 3: material and see what happens when photons of different frequencies 760 00:36:09,600 --> 00:36:12,840 Speaker 3: try to go through it, and what it all means 761 00:36:12,880 --> 00:36:16,320 Speaker 3: about transparency in the universe. But first, let's take another 762 00:36:16,400 --> 00:36:31,640 Speaker 3: quick break. All right, we're talking about transparency, and something 763 00:36:31,680 --> 00:36:34,080 Speaker 3: that comes to mind is, I don't know if you 764 00:36:34,080 --> 00:36:36,319 Speaker 3: read old comic books, or you read comic books when 765 00:36:36,320 --> 00:36:38,319 Speaker 3: you were a kid, there was always an ad in 766 00:36:38,360 --> 00:36:41,800 Speaker 3: the bag for like X ray glasses, and I always wondered, 767 00:36:41,880 --> 00:36:45,120 Speaker 3: like are those for real? Or like how can they 768 00:36:45,120 --> 00:36:47,520 Speaker 3: sell something so bogus out of what's going on? I 769 00:36:47,520 --> 00:36:49,360 Speaker 3: always wanted to order one, but I couldn't because I 770 00:36:49,400 --> 00:36:51,680 Speaker 3: wasn't in Panama. Do you know what I'm talking about? 771 00:36:51,719 --> 00:36:53,240 Speaker 3: Do you know what they were actually selling? 772 00:36:53,400 --> 00:36:56,319 Speaker 1: Do know those ads? And I also wanted those and 773 00:36:56,360 --> 00:36:58,719 Speaker 1: I wanted them to be real, but I also never 774 00:36:58,760 --> 00:37:02,000 Speaker 1: bought them because I was pretty sure they were bogus. 775 00:37:02,440 --> 00:37:04,919 Speaker 1: I mean, you can see through things with X rays, 776 00:37:04,960 --> 00:37:08,440 Speaker 1: and we'll talk about why that happens, why high energy 777 00:37:08,440 --> 00:37:12,279 Speaker 1: photons from X rays can pass through materials sometimes when 778 00:37:12,320 --> 00:37:15,400 Speaker 1: lower energy photons can't. But those glasses can't let you 779 00:37:15,480 --> 00:37:18,839 Speaker 1: see X rays, and they definitely don't generate X rays, right, 780 00:37:18,960 --> 00:37:22,480 Speaker 1: You're not shooting X rays through stuff. So I'm pretty 781 00:37:22,520 --> 00:37:24,560 Speaker 1: sure it was totally bogus. 782 00:37:24,520 --> 00:37:27,600 Speaker 3: Or maybe not. I don't know. We can't say for sure. 783 00:37:27,800 --> 00:37:29,839 Speaker 1: Well, folks out there, if you bought those X ray 784 00:37:29,840 --> 00:37:32,040 Speaker 1: glasses and they did let you see through things, please 785 00:37:32,080 --> 00:37:33,239 Speaker 1: write to us and let us hear. 786 00:37:34,400 --> 00:37:37,360 Speaker 3: Those people are probably rich from a you know, stealing 787 00:37:37,880 --> 00:37:40,080 Speaker 3: a bank vaults and things like that. All right, So 788 00:37:40,120 --> 00:37:42,960 Speaker 3: we're talking about transparency. And you know, when you put 789 00:37:43,000 --> 00:37:45,439 Speaker 3: a bunch of adoms together in the material, they form 790 00:37:45,520 --> 00:37:48,839 Speaker 3: this kind of extended fuzzy cloud of electrons that might 791 00:37:49,200 --> 00:37:52,960 Speaker 3: blocklide or not. And so whether a photon gets through 792 00:37:52,960 --> 00:37:55,320 Speaker 3: that depends on its energy. If it has the energy 793 00:37:55,400 --> 00:37:58,799 Speaker 3: that the electrons in that material like, then it gets 794 00:37:58,840 --> 00:38:01,840 Speaker 3: it's going to get absorbed right and not go through exactly. 795 00:38:01,880 --> 00:38:05,359 Speaker 1: So the basic picture is the same photon approaches this 796 00:38:05,480 --> 00:38:08,840 Speaker 1: now grid of atoms and if it finds an electron 797 00:38:08,880 --> 00:38:11,480 Speaker 1: that can accept its energy, if the electron can go 798 00:38:11,560 --> 00:38:14,359 Speaker 1: from its current quantum state to an allowed quantum state, 799 00:38:14,640 --> 00:38:17,040 Speaker 1: it will absorb that photon. But the picture of the 800 00:38:17,160 --> 00:38:20,040 Speaker 1: energy levels is different from a single atom than with 801 00:38:20,120 --> 00:38:21,960 Speaker 1: the grid of atoms, and the single atom you had 802 00:38:21,960 --> 00:38:24,759 Speaker 1: the ladder, there was sort of sharper energy levels in 803 00:38:24,840 --> 00:38:27,759 Speaker 1: the grid of atoms. Now you have these bands of 804 00:38:27,880 --> 00:38:29,640 Speaker 1: energy levels, and you might think, oh, that makes it 805 00:38:29,640 --> 00:38:32,480 Speaker 1: possible for the electron to absorb basically any photon. It's 806 00:38:32,520 --> 00:38:35,000 Speaker 1: a little bit more complicated than that because we discovered 807 00:38:35,040 --> 00:38:37,200 Speaker 1: that there are these gaps in the energy levels. It's 808 00:38:37,239 --> 00:38:40,160 Speaker 1: not like any possible energy level is allowed for an 809 00:38:40,200 --> 00:38:42,239 Speaker 1: electron in these materials, the way it is for an 810 00:38:42,239 --> 00:38:45,720 Speaker 1: electron in free space. There are still electron energy levels 811 00:38:45,719 --> 00:38:48,920 Speaker 1: that are not allowed. So there's this band of electron 812 00:38:49,040 --> 00:38:52,040 Speaker 1: energies called the valiance band, where the electrons mostly hang 813 00:38:52,120 --> 00:38:54,640 Speaker 1: out in a random material, and then there's a band 814 00:38:54,680 --> 00:38:58,000 Speaker 1: of energies called the conduction band, where electrons can move 815 00:38:58,040 --> 00:39:00,640 Speaker 1: around really freely from atom to adm them and there's 816 00:39:00,680 --> 00:39:04,440 Speaker 1: sometimes a gap between them where electrons can't be and 817 00:39:04,480 --> 00:39:07,880 Speaker 1: that can prevent electrons from absorbing energy of a passing photon. 818 00:39:08,120 --> 00:39:10,920 Speaker 3: M you're saying, like, you can have a material that 819 00:39:11,480 --> 00:39:15,879 Speaker 3: accepts or lets through lights of a certain range of frequencies, 820 00:39:16,239 --> 00:39:18,680 Speaker 3: then it doesn't let them through, and then it does 821 00:39:18,800 --> 00:39:21,279 Speaker 3: for a different range of frequencies, it does lead light. 822 00:39:21,239 --> 00:39:24,160 Speaker 1: Through exactly just like with the atom. It can absorb 823 00:39:24,239 --> 00:39:27,680 Speaker 1: some frequencies and not other frequencies for a grid of atoms. 824 00:39:27,719 --> 00:39:31,319 Speaker 1: For a whole solid material, it can absorb some frequencies 825 00:39:31,520 --> 00:39:34,120 Speaker 1: frequencies where it can hit the electron and jump it 826 00:39:34,280 --> 00:39:37,880 Speaker 1: over this gap between the bands, and it can't absorb 827 00:39:37,920 --> 00:39:41,040 Speaker 1: photons of other frequencies, photons that don't have enough energy 828 00:39:41,200 --> 00:39:44,480 Speaker 1: to get the electrons from one band to another. And 829 00:39:44,520 --> 00:39:47,360 Speaker 1: so different kind of materials have a different sized gap 830 00:39:47,520 --> 00:39:50,040 Speaker 1: between these bands, and so in solid state physics they 831 00:39:50,080 --> 00:39:52,880 Speaker 1: call this the band gap, right, the gap between the 832 00:39:52,920 --> 00:39:56,239 Speaker 1: typical energy levels of the electron and the conduction band 833 00:39:56,280 --> 00:39:58,960 Speaker 1: where electrons are good at like flowing, and some kind 834 00:39:58,960 --> 00:40:02,319 Speaker 1: of materials like meta have a very very small band gap. 835 00:40:02,440 --> 00:40:05,040 Speaker 1: The conduction band is basically right on top of the 836 00:40:05,120 --> 00:40:08,520 Speaker 1: valiance band. There's basically no gap there, and so electrons 837 00:40:08,520 --> 00:40:11,439 Speaker 1: are very good at absorbing photons of a huge range 838 00:40:11,480 --> 00:40:14,279 Speaker 1: of energies because there's a huge spectrum there and other 839 00:40:14,360 --> 00:40:16,920 Speaker 1: materials is a big gap. And in order for an 840 00:40:16,920 --> 00:40:20,200 Speaker 1: electron to absorb a photon, it has to have enough energy, 841 00:40:20,440 --> 00:40:22,640 Speaker 1: and lots of photons just don't have enough energy, and 842 00:40:22,680 --> 00:40:26,160 Speaker 1: so the photons would pass right through the material without interacting. 843 00:40:27,000 --> 00:40:29,520 Speaker 3: Now, when you're talking about light and energy, the light 844 00:40:29,760 --> 00:40:33,399 Speaker 3: of a particular photon is related to its frequency, right 845 00:40:33,480 --> 00:40:35,960 Speaker 3: mostly to it almost or everything to its frequency, and 846 00:40:36,000 --> 00:40:38,160 Speaker 3: so you're really talking about its color, right. 847 00:40:38,120 --> 00:40:40,279 Speaker 1: Yeah, exactly. The energy of a photon doesn't relate to 848 00:40:40,320 --> 00:40:42,840 Speaker 1: its speed. Right. When we think about the energy of 849 00:40:42,840 --> 00:40:45,080 Speaker 1: an electron, we think about its speed, but photons are 850 00:40:45,080 --> 00:40:48,160 Speaker 1: all moving at the same speed. The thing that differentiates 851 00:40:48,200 --> 00:40:52,080 Speaker 1: a high and low energy electron is its frequency, how 852 00:40:52,120 --> 00:40:55,719 Speaker 1: fast the electromagnetic fields are wiggling. And as you say 853 00:40:55,719 --> 00:40:59,600 Speaker 1: that frequency we interpret as color. The photons themselves don't 854 00:40:59,600 --> 00:41:01,640 Speaker 1: have color. It's not like a photon is a red 855 00:41:01,640 --> 00:41:04,920 Speaker 1: photon or a green photon just has a certain frequency 856 00:41:05,160 --> 00:41:07,520 Speaker 1: when it hits our eyeballs our brains give us the 857 00:41:07,600 --> 00:41:10,000 Speaker 1: experience of red or green or blue or whatever, and 858 00:41:10,040 --> 00:41:13,520 Speaker 1: that's a whole philosophical question. But yeah, we associate colors 859 00:41:13,560 --> 00:41:14,640 Speaker 1: with certain frequencies. 860 00:41:14,800 --> 00:41:18,200 Speaker 3: Yes, but we don't talk about philosophy here. But like that, 861 00:41:18,400 --> 00:41:21,719 Speaker 3: if a photon has a certain frequency, it is a 862 00:41:21,760 --> 00:41:24,279 Speaker 3: red photon, right, Like to our eyes it would read 863 00:41:24,320 --> 00:41:24,960 Speaker 3: as red. 864 00:41:24,760 --> 00:41:27,560 Speaker 1: It would read as red. Yeah, And some photons are 865 00:41:27,600 --> 00:41:29,880 Speaker 1: above the visible spectrum, and so we say they're X 866 00:41:29,960 --> 00:41:34,160 Speaker 1: ray photons, or their gamma rays, or their UV photons, right, 867 00:41:34,200 --> 00:41:36,440 Speaker 1: So we can give names to the different parts of 868 00:41:36,440 --> 00:41:39,040 Speaker 1: the frequency spectrum. Some of them we give them colors, 869 00:41:39,040 --> 00:41:41,760 Speaker 1: some of them we just give them labels. Radio waves, 870 00:41:41,800 --> 00:41:46,360 Speaker 1: for example, our photons a very very long frequency, well 871 00:41:46,400 --> 00:41:49,439 Speaker 1: below what we can see, even below the infrared. 872 00:41:49,280 --> 00:41:52,760 Speaker 3: Right, And so that determines whether or not a materials 873 00:41:52,800 --> 00:41:55,960 Speaker 3: transparent to different kinds of light. Because X rays sort 874 00:41:55,960 --> 00:41:59,040 Speaker 3: of lead you see through your body and your bones, right. 875 00:41:59,080 --> 00:42:02,000 Speaker 3: That's because they have a high energy and the electrons 876 00:42:02,040 --> 00:42:04,400 Speaker 3: in your body can absorb them, so they sort of 877 00:42:04,400 --> 00:42:04,719 Speaker 3: go through. 878 00:42:04,880 --> 00:42:08,200 Speaker 1: Yeah. Really interestingly, X rays can pass through the soft 879 00:42:08,239 --> 00:42:11,760 Speaker 1: tissues of your body, but they can't pass through your bones, 880 00:42:12,120 --> 00:42:14,040 Speaker 1: which is why when you see an X ray, what 881 00:42:14,080 --> 00:42:16,719 Speaker 1: you're looking at is basically only the bones. Because that's 882 00:42:16,760 --> 00:42:19,719 Speaker 1: the thing that the X rays didn't pass through, so 883 00:42:19,760 --> 00:42:22,400 Speaker 1: it passes through everything else. Your body is transparent to 884 00:42:22,520 --> 00:42:25,200 Speaker 1: X rays except for your bones. That's why you can 885 00:42:25,200 --> 00:42:27,360 Speaker 1: tell the difference between the bones and the not bones 886 00:42:27,400 --> 00:42:29,000 Speaker 1: part on the X ray. 887 00:42:29,200 --> 00:42:31,920 Speaker 3: Now is that because the I guess bones are made 888 00:42:31,960 --> 00:42:35,080 Speaker 3: out of the different material than my muscles, and so 889 00:42:35,400 --> 00:42:38,680 Speaker 3: my muscles don't absorb X rays what my bones do 890 00:42:38,800 --> 00:42:44,040 Speaker 3: because of the you know, the bonds between the atoms exactly. 891 00:42:44,080 --> 00:42:47,120 Speaker 1: It's the band gap of the material that determines whether 892 00:42:47,239 --> 00:42:51,160 Speaker 1: or not you can absorb photons of a specific frequency. So, 893 00:42:51,239 --> 00:42:53,799 Speaker 1: for example, in a conductor like a metal, like a 894 00:42:53,840 --> 00:42:56,680 Speaker 1: sheet of steel, the band gap is really really small. 895 00:42:56,800 --> 00:42:58,920 Speaker 1: It's very easy to get an electron up into that 896 00:42:58,920 --> 00:43:01,040 Speaker 1: conduction band where it can low around. And that's why 897 00:43:01,080 --> 00:43:04,239 Speaker 1: these things conduct electricity very easily because it's easy to 898 00:43:04,280 --> 00:43:07,640 Speaker 1: have electrons that slide around in the material. So conductor 899 00:43:07,760 --> 00:43:09,759 Speaker 1: like a metal, right, it's really easy to get those 900 00:43:09,800 --> 00:43:12,840 Speaker 1: electrons flying around. It also means it's easy to absorb 901 00:43:13,280 --> 00:43:16,400 Speaker 1: that energy. So that's why things like metals and conductors 902 00:43:16,760 --> 00:43:20,200 Speaker 1: are good at conducting electricity and good at absorbing photons 903 00:43:20,239 --> 00:43:22,840 Speaker 1: and bad at being see through. So that's why a 904 00:43:22,920 --> 00:43:24,520 Speaker 1: sheet of metal, for example, is not. 905 00:43:24,680 --> 00:43:29,359 Speaker 3: Transparent unless it's I guess a wire mesh. 906 00:43:29,080 --> 00:43:32,400 Speaker 1: Yeah, exactly, unless it's a screen, which is why if 907 00:43:32,400 --> 00:43:34,600 Speaker 1: you remember like Star Trek and they had like transparent 908 00:43:34,680 --> 00:43:37,799 Speaker 1: aluminium in Star Trek four or whatever, I always like, well, 909 00:43:37,840 --> 00:43:39,920 Speaker 1: you can't really do that. That is not something we 910 00:43:40,000 --> 00:43:42,759 Speaker 1: know how to do. Although you know, far future societies 911 00:43:42,800 --> 00:43:43,799 Speaker 1: maybe they figured it out. 912 00:43:43,880 --> 00:43:46,520 Speaker 3: I'm not sure I'm familiar with that level of trivia 913 00:43:46,640 --> 00:43:47,760 Speaker 3: for the Story Trek movies. 914 00:43:47,880 --> 00:43:48,279 Speaker 5: But I. 915 00:43:49,960 --> 00:43:52,000 Speaker 1: Remember they got the whales and they had to build 916 00:43:52,000 --> 00:43:53,840 Speaker 1: an aquarium for the whales, and how are they going 917 00:43:53,880 --> 00:43:54,719 Speaker 1: to hold all this water? 918 00:43:54,920 --> 00:43:57,719 Speaker 3: I remember the whales? Yeah, right, but I guess what 919 00:43:57,760 --> 00:44:00,719 Speaker 3: do you mean though, Like aluminium is not it's transparent 920 00:44:00,760 --> 00:44:02,960 Speaker 3: to visible light, but it is still transparent to other 921 00:44:03,040 --> 00:44:05,400 Speaker 3: kinds of light, right, like X rays sort of go 922 00:44:05,480 --> 00:44:08,520 Speaker 3: through metal. No, or do metals like block all light. 923 00:44:08,800 --> 00:44:11,879 Speaker 1: It's always the case that it depends on the frequency, right, 924 00:44:11,920 --> 00:44:14,040 Speaker 1: and so you have to have the right frequency to 925 00:44:14,320 --> 00:44:17,399 Speaker 1: match the energy levels that the object can absorb. If 926 00:44:17,400 --> 00:44:20,920 Speaker 1: you have a huge amount of energy, then probably you're 927 00:44:20,920 --> 00:44:23,359 Speaker 1: going to knock the electrons out of the material. Right, 928 00:44:23,360 --> 00:44:25,840 Speaker 1: then we're getting into the case of like the photoelectric effect. 929 00:44:26,200 --> 00:44:30,239 Speaker 1: So you zap like gamma rays against aluminum, then there's 930 00:44:30,239 --> 00:44:31,960 Speaker 1: definitely going to be an interaction there, but it's going 931 00:44:32,040 --> 00:44:34,440 Speaker 1: to knock the whole electron out of the material. It's 932 00:44:34,480 --> 00:44:37,040 Speaker 1: not just going to push it up to some energy level. 933 00:44:37,160 --> 00:44:38,959 Speaker 1: So at some point this picture breaks down. 934 00:44:39,200 --> 00:44:42,840 Speaker 3: Oh what So at some point you have enough energy 935 00:44:42,880 --> 00:44:45,359 Speaker 3: where the electron JA doesn't someone to say in any groove, 936 00:44:45,400 --> 00:44:46,680 Speaker 3: it just flies out into space. 937 00:44:46,719 --> 00:44:49,640 Speaker 1: Really, yeah, exactly, you can shine light on metal and 938 00:44:49,760 --> 00:44:53,160 Speaker 1: boil off electrons if you have enough energy. It's like 939 00:44:53,160 --> 00:44:56,239 Speaker 1: a highest level band and above that then electrons are 940 00:44:56,280 --> 00:44:59,040 Speaker 1: just free. Again, you've like broken it out of physics jail. 941 00:45:00,239 --> 00:45:03,839 Speaker 3: You get to pay two hundred bucks. Though exactly, Now, 942 00:45:04,239 --> 00:45:06,560 Speaker 3: what happens on the other spectrum, Like what if a 943 00:45:06,600 --> 00:45:10,680 Speaker 3: photon has too little energy like a super infrared or 944 00:45:10,719 --> 00:45:13,360 Speaker 3: something like that, or radio wave as you said, that 945 00:45:13,440 --> 00:45:15,879 Speaker 3: still goes through metal and other things. 946 00:45:15,960 --> 00:45:18,200 Speaker 1: Right, No, radio waves do not go through metal. Right. 947 00:45:18,239 --> 00:45:20,839 Speaker 1: That's why, for example, your phone call is dropped if 948 00:45:20,840 --> 00:45:24,080 Speaker 1: you're in an elevator because metal is like a Faraday cage. 949 00:45:24,080 --> 00:45:28,279 Speaker 1: It will block radio waves even classically, right, the electrons 950 00:45:28,320 --> 00:45:31,000 Speaker 1: in the material will reorganize themselves to cancel out an 951 00:45:31,040 --> 00:45:33,279 Speaker 1: electric field. But from a quantum mechanical point of view, 952 00:45:33,280 --> 00:45:36,720 Speaker 1: a conductor can absorb very very low energy photons because 953 00:45:36,760 --> 00:45:39,120 Speaker 1: the gap there is very small, and so it can 954 00:45:39,160 --> 00:45:41,560 Speaker 1: absorb very very low energy photons. 955 00:45:41,680 --> 00:45:44,040 Speaker 3: But I guess what's going on there though, Like, if 956 00:45:44,040 --> 00:45:46,200 Speaker 3: it's just a single atom and I have an electron orbiting, 957 00:45:46,760 --> 00:45:50,200 Speaker 3: if the forodan has very little energy, I'm going to 958 00:45:50,239 --> 00:45:51,560 Speaker 3: ignore it too, are I? 959 00:45:51,719 --> 00:45:54,400 Speaker 1: Yeah, absolutely you are. In the case of an individual atom, 960 00:45:54,840 --> 00:45:57,480 Speaker 1: then there are photons that have too low an energy 961 00:45:57,520 --> 00:46:00,480 Speaker 1: to move the electron up from level one, level two, 962 00:46:00,600 --> 00:46:03,520 Speaker 1: or level seven to level eight exactly. That can happen 963 00:46:03,920 --> 00:46:06,200 Speaker 1: in a solid. Now you have a whole spectrum of 964 00:46:06,280 --> 00:46:09,480 Speaker 1: energy levels, and so there's lots of very very fine 965 00:46:09,480 --> 00:46:14,080 Speaker 1: gradations allowed there. So materials can absorb low energy electrons 966 00:46:14,080 --> 00:46:16,560 Speaker 1: because there's a very very fine mesh of energy levels. 967 00:46:16,680 --> 00:46:18,919 Speaker 3: Is there a bottom limit there? Like, is there an 968 00:46:19,040 --> 00:46:23,319 Speaker 3: energy for my photon for which it's even outside of 969 00:46:23,360 --> 00:46:26,680 Speaker 3: the gap of material with lots of electrons. 970 00:46:26,160 --> 00:46:28,759 Speaker 1: There might be a lower limit there. I mean, even 971 00:46:28,800 --> 00:46:32,080 Speaker 1: conductors do have some kind of a band gap, so 972 00:46:32,160 --> 00:46:34,719 Speaker 1: you might need a minimum energy to get them up 973 00:46:35,000 --> 00:46:38,279 Speaker 1: from the valiance band to the conduction band, and there 974 00:46:38,360 --> 00:46:41,800 Speaker 1: might even be a limit within those bands a minimum energy, 975 00:46:41,960 --> 00:46:44,279 Speaker 1: so the other might be a limit. Very very low 976 00:46:44,400 --> 00:46:47,560 Speaker 1: energy photons could be ignored even by conductors. But the 977 00:46:47,600 --> 00:46:50,800 Speaker 1: other side of the coin are materials like insulators. Take glass, 978 00:46:50,800 --> 00:46:54,440 Speaker 1: for example. Glass is not a conductor because it has 979 00:46:54,480 --> 00:46:57,080 Speaker 1: a large gap between these energy levels. So mostly electrons 980 00:46:57,120 --> 00:46:58,879 Speaker 1: in the glass are not free to move around. They're 981 00:46:58,880 --> 00:47:02,160 Speaker 1: mostly stuck to the atom that they are around, and 982 00:47:02,200 --> 00:47:03,799 Speaker 1: there's a band gap there. If you want to push 983 00:47:03,800 --> 00:47:06,400 Speaker 1: an electron up to the next energy level, there's like 984 00:47:06,440 --> 00:47:09,000 Speaker 1: a big gap between the energy levels that's normally in 985 00:47:09,320 --> 00:47:11,959 Speaker 1: and the first one that's available. It's like a few 986 00:47:12,040 --> 00:47:15,840 Speaker 1: electron bolts, and so photons that hit glass in the 987 00:47:15,920 --> 00:47:19,759 Speaker 1: visible spectrum mostly do not have enough energy to get 988 00:47:19,800 --> 00:47:22,680 Speaker 1: the electron up to the conduction band. And so that's 989 00:47:22,719 --> 00:47:26,919 Speaker 1: why visible light photons do pass through glass. They pass 990 00:47:27,000 --> 00:47:29,799 Speaker 1: right through this whole grid of atoms and all those electrons, 991 00:47:30,000 --> 00:47:32,480 Speaker 1: but they don't have enough energy to move the electrons 992 00:47:32,719 --> 00:47:35,000 Speaker 1: up to the next band, and so they're ignored and 993 00:47:35,040 --> 00:47:36,040 Speaker 1: they pass right through. 994 00:47:36,320 --> 00:47:38,880 Speaker 3: You mean like a material like glass. It's like the 995 00:47:38,920 --> 00:47:42,120 Speaker 3: atoms are basically it's just a bunch of individual atoms 996 00:47:42,160 --> 00:47:45,400 Speaker 3: hanging out together. They're not sharing a lot of electrons, 997 00:47:45,480 --> 00:47:47,720 Speaker 3: which is what you need to make a good conductor. 998 00:47:47,760 --> 00:47:50,520 Speaker 3: They're mostly just doing what they would do normally on 999 00:47:50,560 --> 00:47:53,280 Speaker 3: their own, and so you have a very limited number 1000 00:47:53,360 --> 00:47:54,920 Speaker 3: of frequencies that it blocks. 1001 00:47:55,000 --> 00:47:57,040 Speaker 1: The picture is a little bit more complicated. I mean 1002 00:47:57,080 --> 00:47:59,520 Speaker 1: the glass atoms still do interact with each other, so 1003 00:47:59,520 --> 00:48:03,000 Speaker 1: they do this band of energy levels for the electrons 1004 00:48:03,239 --> 00:48:05,520 Speaker 1: because they are bonded together, right, I mean, glass is 1005 00:48:05,560 --> 00:48:07,920 Speaker 1: not a crystal, but still there are bonds between the 1006 00:48:07,960 --> 00:48:10,680 Speaker 1: atoms they are interacting, so there is a spectrum of 1007 00:48:10,800 --> 00:48:13,600 Speaker 1: energy levels the electrons can be in For a glass 1008 00:48:13,920 --> 00:48:15,880 Speaker 1: that's not just like an atom. It's not just like 1009 00:48:15,920 --> 00:48:19,879 Speaker 1: a sharp layer, but it's mostly a full band of electrons. 1010 00:48:19,920 --> 00:48:22,160 Speaker 1: But that band is mostly filled. Then the electrons can't 1011 00:48:22,160 --> 00:48:24,320 Speaker 1: really go anywhere. It's like if you're on a plane, 1012 00:48:24,480 --> 00:48:27,040 Speaker 1: every seat is taken, then you can't like move from 1013 00:48:27,120 --> 00:48:29,799 Speaker 1: seat to seat, and the next energy level above that 1014 00:48:29,920 --> 00:48:31,840 Speaker 1: is kind of high. Right, you can't like get to 1015 00:48:31,880 --> 00:48:33,799 Speaker 1: first class. You need a lot of energy to get 1016 00:48:33,800 --> 00:48:37,360 Speaker 1: to first class in a glass, and so everybody's basically 1017 00:48:37,400 --> 00:48:40,359 Speaker 1: stuck in their seat in coach, and the electrons can't 1018 00:48:40,360 --> 00:48:43,360 Speaker 1: really absorb little amounts of energy. They need a lot 1019 00:48:43,400 --> 00:48:46,200 Speaker 1: of energy to get promoted up to first class, which 1020 00:48:46,200 --> 00:48:49,320 Speaker 1: is the next band of energy levels. In a glass 1021 00:48:49,360 --> 00:48:51,719 Speaker 1: and a conductor, that band is much much lower, so 1022 00:48:51,760 --> 00:48:53,800 Speaker 1: it doesn't take as much energy to get up there, 1023 00:48:54,000 --> 00:48:56,560 Speaker 1: and glass the band is really large, it's really hard 1024 00:48:56,600 --> 00:48:59,360 Speaker 1: to get promoted up to the next set of energy levels. 1025 00:49:00,040 --> 00:49:03,640 Speaker 3: I think what you're saying is that glass is transparent 1026 00:49:03,840 --> 00:49:07,200 Speaker 3: for a wider range of frequencies of light, which just 1027 00:49:07,280 --> 00:49:10,640 Speaker 3: happened to be in our visible spectrum. But glass is opaque. 1028 00:49:10,920 --> 00:49:13,839 Speaker 3: It is not transparent to certain frequencies of light. 1029 00:49:14,040 --> 00:49:17,719 Speaker 1: That's exactly right. For example ultraviolet, right, ultraviolet is light 1030 00:49:17,800 --> 00:49:20,960 Speaker 1: with higher energy. You can't see ultraviolet light. It's the 1031 00:49:21,040 --> 00:49:22,759 Speaker 1: kind that's going to give you a sunburn or can 1032 00:49:22,840 --> 00:49:25,440 Speaker 1: hurt your eyeballs. But it has more energy, and it 1033 00:49:25,520 --> 00:49:28,960 Speaker 1: has enough energy to bump one of these electrons up 1034 00:49:29,120 --> 00:49:32,320 Speaker 1: over this gap into the conduction band, and so glass 1035 00:49:32,440 --> 00:49:35,320 Speaker 1: can absorb uv photons. That's why you don't get a 1036 00:49:35,360 --> 00:49:39,120 Speaker 1: sunburn if you're sunbathing through glass, glass is like sunscreen. 1037 00:49:39,320 --> 00:49:42,680 Speaker 3: Wait what I can just put a glass over me 1038 00:49:42,800 --> 00:49:45,520 Speaker 3: and I'm going to give you sunburn? Is that a 1039 00:49:45,560 --> 00:49:46,879 Speaker 3: solid medical advice there? 1040 00:49:47,000 --> 00:49:48,480 Speaker 1: That is not solid medical advice? 1041 00:49:48,680 --> 00:49:50,719 Speaker 3: And just to be transparent, we're non medical. 1042 00:49:50,400 --> 00:49:54,080 Speaker 1: Doctors, right, that's right. But it does block some of 1043 00:49:54,120 --> 00:49:56,640 Speaker 1: the UV, so it would reduce your sunburn. It still 1044 00:49:56,680 --> 00:50:00,000 Speaker 1: totally advise you to wear sunscreen, but glass is not transparent, 1045 00:50:00,120 --> 00:50:02,759 Speaker 1: and to UV, the same way is to visible light, 1046 00:50:03,080 --> 00:50:04,959 Speaker 1: absorbs a lot more of the UV. 1047 00:50:05,120 --> 00:50:08,240 Speaker 3: Is that kind of what's going on with sunscreens? Like lotion? 1048 00:50:08,440 --> 00:50:08,520 Speaker 5: Right? 1049 00:50:08,640 --> 00:50:08,799 Speaker 7: Right? 1050 00:50:08,840 --> 00:50:11,239 Speaker 3: It has materials that absorb UV. 1051 00:50:11,440 --> 00:50:15,000 Speaker 1: Right, Yeah, exactly, your sunscreen is just opaque to UV. 1052 00:50:15,120 --> 00:50:18,040 Speaker 1: It has stuff in it that could accept those photons 1053 00:50:18,080 --> 00:50:20,680 Speaker 1: and absorb it rather than letting those UV photons pass 1054 00:50:20,719 --> 00:50:22,800 Speaker 1: into your body and then cause damage. 1055 00:50:22,880 --> 00:50:25,480 Speaker 3: Okay, Now, what makes a piece of glass like a 1056 00:50:25,480 --> 00:50:27,840 Speaker 3: piece of red glass or a piece of blue glass? 1057 00:50:27,880 --> 00:50:31,240 Speaker 1: So that often is because of doping. You like change 1058 00:50:31,280 --> 00:50:35,279 Speaker 1: the energy levels of the glass by adding impurities, and 1059 00:50:35,320 --> 00:50:38,319 Speaker 1: so these other molecules change the band gap, so make 1060 00:50:38,360 --> 00:50:41,360 Speaker 1: it possible for different kinds of photons to be absorbed. 1061 00:50:41,880 --> 00:50:45,439 Speaker 3: And so I guess you sort of narrow the band gap, right, 1062 00:50:45,480 --> 00:50:49,320 Speaker 3: like regular glass wide range of photon frequencies that it 1063 00:50:49,400 --> 00:50:51,880 Speaker 3: lets through, but like blue glass I imagine, has a 1064 00:50:52,000 --> 00:50:55,200 Speaker 3: narrow gap where it only lets through light that is 1065 00:50:55,239 --> 00:50:56,280 Speaker 3: bluish for example. 1066 00:50:56,320 --> 00:50:59,200 Speaker 1: That's right. So sometimes people add like aluminium oxide the glass, 1067 00:50:59,480 --> 00:51:02,640 Speaker 1: and that makes glass pink or red because it absorbs 1068 00:51:02,680 --> 00:51:05,359 Speaker 1: the green and the blue photons. And so I actually 1069 00:51:05,400 --> 00:51:08,400 Speaker 1: got an email from a listener, Matt Cleveland, who says, 1070 00:51:08,719 --> 00:51:10,920 Speaker 1: what is it about the photons of sunlight that costs 1071 00:51:10,960 --> 00:51:14,040 Speaker 1: some objects to fade and lose their color? What is 1072 00:51:14,080 --> 00:51:16,640 Speaker 1: it that's breaking down? Why do some objects lose their 1073 00:51:16,640 --> 00:51:19,360 Speaker 1: color from this interaction with the sun's photons and others 1074 00:51:19,520 --> 00:51:19,840 Speaker 1: do not? 1075 00:51:20,880 --> 00:51:23,080 Speaker 3: That's an interesting question. Yeah, Like if you leave your 1076 00:51:23,120 --> 00:51:25,319 Speaker 3: T shirt out in the sun, it's going to get faded, right, 1077 00:51:25,320 --> 00:51:27,200 Speaker 3: It's going to get bleached. That's kind of why your 1078 00:51:27,200 --> 00:51:29,160 Speaker 3: hair also gets bleached. A little bit of view. Stay 1079 00:51:29,160 --> 00:51:30,320 Speaker 3: out in the sun a lot. 1080 00:51:30,280 --> 00:51:33,759 Speaker 1: Yeah, exactly, And that's mostly the UV light, Right. These 1081 00:51:33,840 --> 00:51:36,400 Speaker 1: chemicals absorb UV light, and the UV light has a 1082 00:51:36,440 --> 00:51:40,600 Speaker 1: lot of energy, so sometimes it breaks down those chemicals. Right, 1083 00:51:40,640 --> 00:51:43,799 Speaker 1: we talked about like photons hitting electrons and banging them 1084 00:51:43,800 --> 00:51:46,960 Speaker 1: out of materials well. UV light. Sometimes these atoms can 1085 00:51:47,000 --> 00:51:49,680 Speaker 1: absorb it, but it also damages the atoms the same 1086 00:51:49,719 --> 00:51:52,239 Speaker 1: way like can damage things in your body, and so 1087 00:51:52,360 --> 00:51:55,960 Speaker 1: chemicals in objects can sometimes break down when they absorb 1088 00:51:56,080 --> 00:51:58,720 Speaker 1: UV light. And what you'll notice is that red stuff 1089 00:51:58,920 --> 00:52:03,440 Speaker 1: is especially suceptible to this because they absorb more high energy, 1090 00:52:03,520 --> 00:52:06,640 Speaker 1: more blue and more UV photons. So things that look 1091 00:52:06,680 --> 00:52:09,400 Speaker 1: red are things that absorb in the blue spectrum and 1092 00:52:09,440 --> 00:52:12,920 Speaker 1: therefore absorb more UV light and are likely to fade 1093 00:52:12,920 --> 00:52:15,160 Speaker 1: more in sunlight than things that are. 1094 00:52:15,040 --> 00:52:18,680 Speaker 3: Blue, because if they're red, then that means they're mostly 1095 00:52:18,719 --> 00:52:20,960 Speaker 3: reflecting the red part, but they're absorbing the blue light. 1096 00:52:21,120 --> 00:52:23,920 Speaker 1: Yeah, exactly. And UV is like super blue. 1097 00:52:24,120 --> 00:52:27,040 Speaker 3: But sometimes materials get harder in the sun, right, Like 1098 00:52:27,080 --> 00:52:29,680 Speaker 3: if you leave a piece of rubber or a rubber 1099 00:52:29,719 --> 00:52:32,920 Speaker 3: band or your car tires, they get more brittle as 1100 00:52:32,960 --> 00:52:34,160 Speaker 3: they stay out in the sun longer. 1101 00:52:34,239 --> 00:52:37,200 Speaker 1: Yeah, that's a similar process. You're not changing their transparency, 1102 00:52:37,320 --> 00:52:40,080 Speaker 1: but still the UV light is changing the chemical composition 1103 00:52:40,239 --> 00:52:42,600 Speaker 1: because it's being absorbed and it's breaking down some of 1104 00:52:42,600 --> 00:52:45,800 Speaker 1: the bonds, and it's changing the chemical nature of the substance. 1105 00:52:45,920 --> 00:52:48,200 Speaker 3: So if you put your car inside of a glass house. 1106 00:52:50,440 --> 00:52:52,560 Speaker 3: Then they'll stay the same color and the tires will 1107 00:52:52,560 --> 00:52:53,959 Speaker 3: stay bouncy. Is that what you're saying? 1108 00:52:54,040 --> 00:52:56,480 Speaker 1: Yeah? Or if you smear your car and sunscreen either one. 1109 00:52:56,560 --> 00:52:58,080 Speaker 3: Oh, I guess you have to be careful. You know, 1110 00:52:58,360 --> 00:52:59,800 Speaker 3: you know what to say about cars that live in 1111 00:53:00,080 --> 00:53:04,920 Speaker 3: glass houses. All right, Well, I guess it's an interesting 1112 00:53:05,200 --> 00:53:08,279 Speaker 3: look into a very familiar thing that is all around us, right, 1113 00:53:08,320 --> 00:53:10,160 Speaker 3: Like the screen on your phone is made out of 1114 00:53:10,160 --> 00:53:13,200 Speaker 3: transparent glass, and your windows, and every time you go 1115 00:53:13,239 --> 00:53:15,400 Speaker 3: to the doctor or the dentists and they take X rays, 1116 00:53:15,440 --> 00:53:16,640 Speaker 3: it's like physics going on. 1117 00:53:16,760 --> 00:53:20,600 Speaker 1: Right, There is physics going on everywhere. What's amazing to 1118 00:53:20,640 --> 00:53:23,400 Speaker 1: me is that sometimes we can even unravel this like 1119 00:53:23,560 --> 00:53:27,920 Speaker 1: microphysical picture of what's happening. Do explain our everyday experience. 1120 00:53:28,000 --> 00:53:30,879 Speaker 1: Why things are squishy, why things are hard, why things 1121 00:53:30,880 --> 00:53:34,320 Speaker 1: conduct electricity, why things are see through. It all comes 1122 00:53:34,320 --> 00:53:37,120 Speaker 1: down to what's happening at the atomic level or the 1123 00:53:37,160 --> 00:53:40,880 Speaker 1: subatomic level, And incredibly, that's a story we can sometimes 1124 00:53:40,960 --> 00:53:42,719 Speaker 1: understand and even explain to you. 1125 00:53:43,520 --> 00:53:47,160 Speaker 3: Yes, it's almost like the universe is transparent to science, 1126 00:53:48,400 --> 00:53:51,200 Speaker 3: or it's like scientists have X ray glasses. 1127 00:53:50,840 --> 00:53:53,480 Speaker 1: Or maybe the UV photons of the universe are just 1128 00:53:53,600 --> 00:53:55,160 Speaker 1: frying our brains. 1129 00:53:55,719 --> 00:53:58,279 Speaker 3: Because your skull is made at a glass. What's going 1130 00:53:58,320 --> 00:53:58,640 Speaker 3: on there? 1131 00:53:59,440 --> 00:54:02,200 Speaker 1: I'm going to go put sunscreen on my brain, yeah. 1132 00:54:02,160 --> 00:54:05,480 Speaker 3: Or a hat, you know. They physics have invented hats 1133 00:54:05,520 --> 00:54:08,920 Speaker 3: also which helps with sun damage. 1134 00:54:09,440 --> 00:54:11,759 Speaker 1: Quantum hats. We should sell those all right. 1135 00:54:11,800 --> 00:54:14,200 Speaker 3: Well, we hope you enjoyed that. Thanks for joining us, 1136 00:54:14,840 --> 00:54:15,600 Speaker 3: See you next time. 1137 00:54:23,560 --> 00:54:26,360 Speaker 1: Thanks for listening, and remember that. Daniel and Jorge Explain 1138 00:54:26,440 --> 00:54:30,440 Speaker 1: the Universe is a production of iHeartRadio. For more podcasts 1139 00:54:30,440 --> 00:54:35,080 Speaker 1: from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever 1140 00:54:35,160 --> 00:54:36,880 Speaker 1: you listen to your favorite shows.