Speaker 1: Hey, or hey, have you ever seen a glass frog?
Speaker 2: Mmm?
Speaker 3: You mean one of those transparent amphibians or is it
Speaker 3: like a it'll keep saking me ou out of class?
Speaker 1: I mean they're a real live frog. I was reading
Speaker 1: that they are native to Central America, including Panama, where
Speaker 1: you grew up. Yeah.
Speaker 3: I have heard that, but to be honest, I haven't
Speaker 3: seen one in person. You could probably find them in
Speaker 3: the jungle, but it's not like they're jumping around my house.
Speaker 1: Well did you ever wonder what it would be like
Speaker 1: to be transparent yourself?
Speaker 3: Sounds terrible? I guess you want people to see, right.
Speaker 3: Everyone wants to be seen these.
Speaker 1: Days, unless you want to sneak around the house and
Speaker 1: be invisible.
Speaker 3: I guess if you have the option of turning transparent,
Speaker 3: that's cool. Like, I think all kids dream of being
Speaker 3: invisible at some point.
Speaker 1: I always wondered what happens if you're invisible and you
Speaker 1: take a bite of an apple, Like, can everybody see
Speaker 1: that apple work its way through you? H?
Speaker 3: I guess it depends on you, know, like how the
Speaker 3: invisibility works, Like are you a space that is transparent
Speaker 3: or is it just your molecules are transparent? Or you
Speaker 3: can just eat a cookie mid out of glass frogs
Speaker 3: or eat glass. I guess that sounds less tasty.
Speaker 1: I don't think I want to take a bite out
Speaker 1: of that.
Speaker 3: Yeah, I see you, man, I see you. I am Rhem,
Speaker 3: a cartoonist and the creator of PhD comics.
Speaker 1: Hi, I'm Daniel. I'm a particle physicist and a professor.
Speaker 1: You see Irvine, And even though you can only hear
Speaker 1: my voice, I feel seen.
Speaker 3: Well, that's good. Everyone wants to be seen. I think
Speaker 3: mostly we just feel heard in this podcast.
Speaker 1: That's right. We feel heard, not hurt, because we love
Speaker 1: sharing with you our passion and curiosity about the universe.
Speaker 1: And when I hear back from listeners that's something we
Speaker 1: have said has touched on their deep seated need to
Speaker 1: understand the universe. I do feel heard and seen.
Speaker 3: Yeah, because it is a pretty amazing universe with lots
Speaker 3: to see out there and to hear. I guess if
Speaker 3: you have the right kinds of ears. Because sound doesn't
Speaker 3: travel in space, does it?
Speaker 1: It actually does, just very very slowly.
Speaker 3: Why are you kidding or are you serious?
Speaker 4: No?
Speaker 1: Space is not totally empty, and so in principle, there
Speaker 1: are sound waves that do propagate through the solar wind,
Speaker 1: for example. But in practice, if you're in outer space,
Speaker 1: you're going to freeze before you hear anything.
Speaker 3: Also, you would be hearing it in slow motion, I
Speaker 3: guess right, if sound moves slower, Like, don't go out
Speaker 3: in space too late, you're.
Speaker 1: Dead, that's right. You should shout at your friends in space,
Speaker 1: like a year before they go space walking without a
Speaker 1: sud on so they can hear you.
Speaker 3: Yeah, oh, that's not going to help them. Also, how
Speaker 3: do you shout in space? Like you have to take
Speaker 3: off your helmet to shout, and then that's not good.
Speaker 3: Either in space nobody can hear you. Or you shouldn't
Speaker 3: be hurt or you might get hurt.
Speaker 1: But you should turn your eyeballs up to the night
Speaker 1: sky and wonder how everything works. And you should also
Speaker 1: look down on the ground beneath you and see if
Speaker 1: you can puzzle out the mysteries of everyday objects, the
Speaker 1: nature of our universe, how everything works, how it weaves
Speaker 1: itself together to make this incredible cosmos. That's one of
Speaker 1: the deep mysteries that humans want to unravel. And those
Speaker 1: are the topics we like to take a part on
Speaker 1: this podcast.
Speaker 3: Yeah, and It's amazing and lucky that we can see
Speaker 3: so many things out there in the universe and around us,
Speaker 3: so that we know where they are and we can
Speaker 3: also study them and figure out how they work, what
Speaker 3: they're made out of, and what the rules of this
Speaker 3: crazy universe are. But anyways, welcome to our podcast, Daniel
Speaker 3: and Jorge Explain the Universe, a production of iHeartRadio.
Speaker 1: In which we try to do exactly that, take apart
Speaker 1: the whole universe, see how it works, and explain it
Speaker 1: to you. Some of the questions we love to tackle
Speaker 1: involve the tiniest things in the universe, what the rules
Speaker 1: are for how they work, or the biggest things in
Speaker 1: the universe, like the whole universe itself or super massive
Speaker 1: black holes at the hearts of galaxies. But there's also
Speaker 1: a lot of fascinating physics in between. How those tiny
Speaker 1: little objects pull themselves together to behave in weird and
Speaker 1: wonderful ways. The reason ice cream is so tasty, the
Speaker 1: reason metals conduct electricity, the reason your chair holds you
Speaker 1: up is because of zillions of atoms all working together
Speaker 1: to have fascinating phenomena.
Speaker 3: Yeah, and thankfully we can see them all and hear
Speaker 3: them all so we can study them. It'd be kind
Speaker 3: of hard to know and study the universe if everything
Speaker 3: was invisible, right, or if we didn't have eyeballs.
Speaker 1: In years, that's true, and recently we've discovered the amazing
Speaker 1: fact that most of the universe is invisible. The dark
Speaker 1: matter that's out there is most of the stuff in
Speaker 1: the universe, and we didn't even know it existed until recently.
Speaker 1: Because it's invisible to us. Life passes right through it,
Speaker 1: and there's lots of things going on out there in
Speaker 1: the universe that you just cannot see because your eyeballs
Speaker 1: can't pick them up, you can't taste them, you can't
Speaker 1: smell them. So most of the universe is actually invisible.
Speaker 1: Transparency turns out to be the name of the game.
Speaker 3: So the universe got its kid wish of being invisible.
Speaker 3: Is that what you're saying? What's it trying to sneak
Speaker 3: around and do?
Speaker 1: I don't know, but those dark matter kids are probably
Speaker 1: eating dark matter cookies all night long, and.
Speaker 3: They're made out of dark chocolate and taste better too.
Speaker 3: And my new wish used to be a dark matter person.
Speaker 3: They get to eat dark meat and their chicken.
Speaker 1: Well, It also answers that other question. You know what
Speaker 1: happens when a dark matter kid eats a dark matter cookie.
Speaker 1: Of course it goes through them and becomes dark matter
Speaker 1: on the way out.
Speaker 3: Darker matter, you mean, the darkest matter, because a black hole.
Speaker 3: That's where black holes come from.
Speaker 1: That's exactly right. Black holes are dark matter toilets.
Speaker 3: They're are worse through the things that go into the toilet.
Speaker 1: Here we are elevating the discourse of the nature of
Speaker 1: the universe.
Speaker 3: Yeah, I mean, listeners want to be reached into their
Speaker 3: soul and touched right and seen and hurt.
Speaker 1: I'm not sure they want to have this particular taste.
Speaker 3: However, Well, who said anything about tasting.
Speaker 1: We're talking about eating dark matter cookies. You're fantasizing about
Speaker 1: dark matter chocolate over there. It's all about taste.
Speaker 3: That's why we're all about taste here.
Speaker 1: Good taste, bad taste, you.
Speaker 3: Decide, that's right. We're we're scientists. We explore the full
Speaker 3: range of tastes available to the human experience. But it
Speaker 3: is interesting that even the things that are invisible, we
Speaker 3: can still somehow see them through other things. Right, I mean,
Speaker 3: if dark matter, dark energy was completely invisible. We wouldn't
Speaker 3: even know it was there, but somehow it has an
Speaker 3: influence on things that we can see, thankfully, and that's
Speaker 3: how we know they're there.
Speaker 1: Yes, transparency turns out to be quite a subtle issue.
Speaker 1: Some things can be transparent to one kind of light
Speaker 1: but not to another. Some kinds of light can pass
Speaker 1: through some objects but not other objects. As always, when
Speaker 1: you dig into it, you discover there's a fascinating physics
Speaker 1: story that underlines how things work.
Speaker 3: Yep, this is an interesting topic and so today on
Speaker 3: the podcast we'll be tackling the question how does transpy work?
Speaker 1: Some titled how can your kids eat cookies without you knowing?
Speaker 3: Well, I don't think they need to be transparent to
Speaker 3: do that, depending how sneaky they are and how how
Speaker 3: early are you going to sleep or wake up? Now,
Speaker 3: this is an interesting question, Daniel. I imagine it means
Speaker 3: what makes things see through? Not like how do transparencies
Speaker 3: work or how does government transparency work? Because that apparently
Speaker 3: doesn't work that well or sometimes it works too well.
Speaker 1: That's right. We want sunlight on all the operations of
Speaker 1: the universe, but in particular here we're wondering about why
Speaker 1: light can pass through some kinds of things. Why you
Speaker 1: can see through glass but you can't see through stone.
Speaker 1: Why X rays reveal your soft tissues but not your bones.
Speaker 1: What is the underlying physics? What is the microscopic picture
Speaker 1: of transparency.
Speaker 3: We're exploring the full range of transparent behavior.
Speaker 1: We're just trying to be transparent about how the universe works.
Speaker 3: We're trying to be transparent about how the podcast works,
Speaker 3: is what I'm saying. And to be fully transparent, I
Speaker 3: have not I've read the outlete for today's episode. I
Speaker 3: am kind of winging it today, which is totally different
Speaker 3: from other days.
Speaker 1: Right exactly. If you don't notice the difference between this
Speaker 1: episode and any other episode, then folks, you learn something
Speaker 1: about how the sausage is made.
Speaker 3: You learned that Jorges could thinking done the fly and
Speaker 3: reading really quickly. But anyways, as usually, we were wondering
Speaker 3: how many people out there had thought about things that
Speaker 3: are transparent and if they know how transparency works.
Speaker 1: So thanks very much to everybody who answers these questions.
Speaker 1: It gives us a great insight into what people already
Speaker 1: know and don't know. If you'd like to help us
Speaker 1: out for a future episode of the podcast. Please don't
Speaker 1: be shy write to us two questions at Danielandjorge dot com.
Speaker 3: So think about it for a second. Do you know
Speaker 3: how transparency works? Here's what people had to say.
Speaker 4: Transparency works, in my opinion, by not refracting, or the
Speaker 4: medium that the lightest passing through does not refract in
Speaker 4: any way or change the path of the life. And
Speaker 4: that way you don't get any kind of interference and
Speaker 4: you can see through something it is transparent, so perfectly clear.
Speaker 2: I'm pretty sure that transparency works by like the light
Speaker 2: can travel through the material without being absorbed, so maybe
Speaker 2: there's a lot of spaces between it, and so yeah,
Speaker 2: that light just doesn't get absorbed, so it goes through
Speaker 2: and it's merry way.
Speaker 5: At first, I thought it was simple because the material
Speaker 5: density would lead to more or less transparency, like air
Speaker 5: or some gas and low density would be transparent to
Speaker 5: some point because of the particle density. But then again,
Speaker 5: glass is quite a high density, right, and it's still transparent,
Speaker 5: So maybe it's about reflective index. And yeah, like if
Speaker 5: you put glass in a water that comes invisible because
Speaker 5: it's fully transparent because of the reflective indices, I.
Speaker 6: Would guess transparency has to do with interactions between different
Speaker 6: forces and particles and whether they interact or not. If
Speaker 6: they interact, they are not transparent to each other, and
Speaker 6: if they don't interact, they're transparent to each other.
Speaker 1: It's like a real a race.
Speaker 7: So photon gets emitted, it's absorbed by the first atom
Speaker 7: in the glass, for instance, and then it gets retransmitted
Speaker 7: to the next, and so on and so forth until
Speaker 7: it goes over the other side of the of the
Speaker 7: glass or whatever transparent material we are talking about.
Speaker 8: In order to reflect light, the photon would have to
Speaker 8: be absorbed and then re emitted, I think so. I
Speaker 8: guess in order to be transparent, it would have to
Speaker 8: be something that doesn't absorb photons for whatever reason. That'd
Speaker 8: be interesting one.
Speaker 3: That all right, some pretty intricate answers. I feel like
Speaker 3: we had some hardcore physicists here in the pool today.
Speaker 3: Did you ask people on the internet or in your department?
Speaker 1: These are all Internet answers, and they really reflect the
Speaker 1: incredible complexity of light and matter into it in a
Speaker 1: few episodes, what happens when things reflect? What color means.
Speaker 1: But there really is a whole lot going on here.
Speaker 1: Light bouncing off of matter, or passing through matter, or
Speaker 1: refracting through matter is really a very complicated phenomenon, very
Speaker 1: tricky to understand from the microphysics point of view.
Speaker 3: Yes, I like how it reflects how smarter listeners are.
Speaker 1: But so many really interesting answers, a lot of which
Speaker 1: really get at the heart of the question, which is
Speaker 1: that it's about the interactions of the photons with the material.
Speaker 1: There's also some misunderstanding there, like the idea that light
Speaker 1: goes through things if there are spaces for it to
Speaker 1: like wiggle its way through. That's not really the way
Speaker 1: that we think about it, but we'll dig into it
Speaker 1: and explain it.
Speaker 3: I guess it depends, like the difference between transparency and
Speaker 3: air and transparency in glass and transparency through different kinds
Speaker 3: of light, right, it sort of depends.
Speaker 1: There are definitely differences between air and glass, but the
Speaker 1: basic physics of transparency is quite similar in either case.
Speaker 1: Are the photons like avoiding the atoms, It's not like
Speaker 1: they're fine a pathway through. That's maybe the way transparency
Speaker 1: works for like a screen door. You can see through
Speaker 1: a screen because there are literally holes that a photon
Speaker 1: can pass through. But the reason you can see glass
Speaker 1: is not because it's like a microscreen. That glass is
Speaker 1: finding tiny little holes in the glass to wiggle its
Speaker 1: way through.
Speaker 3: I guess we'll dig into that. And so let's jump
Speaker 3: right in, Daniel, what are the basics what makes something transparent?
Speaker 1: So first let's clarify what we mean by transparent. Right,
Speaker 1: by transparent really just mean that you can see through it.
Speaker 1: It means if you shine a light from one side,
Speaker 1: the light comes out the other side with basically no scattering,
Speaker 1: Like the light is still coherent. If you have an
Speaker 1: image of an ice cream cone on one side, you're
Speaker 1: going to see the ice cream cone still on the
Speaker 1: other side. It might be refracted a little bit or
Speaker 1: bent or distorted, but it's transparent if those images are preserved.
Speaker 3: I see. So like, if light can pass through it
Speaker 3: basically is what it means to be transparent, or information
Speaker 3: from light passes through it without any distortions exactly.
Speaker 1: And even glass, which we think of as transparent, does
Speaker 1: distort it a little bit. Like if you stick your
Speaker 1: finger behind a pane of glass and only half of
Speaker 1: your finger is sticking behind it, you'll notice the finger
Speaker 1: no longer looks like a whole finger. It's like broken
Speaker 1: in half because the path of the light through the
Speaker 1: glass changes a little bit relative to the path of
Speaker 1: the light through the air. That's refraction, which is a
Speaker 1: whole other complicated topic that we dug into in another episode.
Speaker 1: But still you can see your finger right the same
Speaker 1: way you can see through water and you can see
Speaker 1: through air. You can identify things. You can recognize things
Speaker 1: even if the path of the light is slightly.
Speaker 3: Changed, all right. So that means that a light can
Speaker 3: or at least information that the light had before it
Speaker 3: went through the material makes it through unscales. That's what
Speaker 3: it means to be transparent.
Speaker 1: Yeah, And the fundamental thing that's happening there relates to
Speaker 1: how photons either interact or don't interact with the electrons
Speaker 1: in that material. And to understand that, we have to
Speaker 1: understand a little bit about the quantum mechanics of those electrons,
Speaker 1: what energy is they're allowed, and how photons decide whether
Speaker 1: or not to interact with those electrons.
Speaker 3: And you were saying, it's not like it's a screen
Speaker 3: door where like some other light is finding a pass
Speaker 3: through it, But that can also happen, Kennet, Like if
Speaker 3: you have something really thin maybe, or something really sparse
Speaker 3: or light like air, there has to be some photons
Speaker 3: that are making it through without interacting with anything. Right.
Speaker 1: Well, most materials, even like a thin sheet of paper,
Speaker 1: are dense enough that photons are not like finding holes
Speaker 1: in them. The reason that light gets through is because
Speaker 1: the photons are ignoring the atoms. It's not that they're
Speaker 1: missing them, it's that they're passing through without interacting. Because
Speaker 1: when a photon passes by an atom, it doesn't always
Speaker 1: interact with it. There are rules about whether or not
Speaker 1: photons can interact with atoms, and in a solid it's
Speaker 1: complicated because you have lots and lots of atoms. But
Speaker 1: the easiest way to understand it is to start with
Speaker 1: an individual atom. Just think about like a single hydrogen
Speaker 1: atom in space and you shoot a photon at it
Speaker 1: is that photon going to get absorbed by the atom
Speaker 1: or is the photon going to ignore the atom? And
Speaker 1: that depends on the energy of the photon, because these
Speaker 1: atoms can only absorb photon of certain energies.
Speaker 3: Well, I guess it also maybe depends on where you
Speaker 3: shoot the photon, right, Like, if I have an hydrogen atom,
Speaker 3: floating out there in the air and I shoot a
Speaker 3: laser beam, you know, a mile to one side of it.
Speaker 3: It's not going to interact with the hydrogen, is it.
Speaker 1: No, that's right. But if you have a huge wall
Speaker 1: of hydrogen atoms and you shoot the laser beam at it,
Speaker 1: then it's not going to be able to avoid the hydrogen, right,
Speaker 1: It's going to have to get through the other side.
Speaker 1: It's going to have to pass through the hydrogen. And
Speaker 1: that's what makes things transparent. I mean, one thing is
Speaker 1: to have holes in it. Sure, where you can see
Speaker 1: through a screen door, even if it's made of metal
Speaker 1: or stone, as long as there are holes in it.
Speaker 1: But I think the physics of transparency is more interesting
Speaker 1: if we think about what happens when light ignores the material,
Speaker 1: if it passes through the material, rather than finding holes
Speaker 1: or ways around it.
Speaker 3: Well, I know it's more interesting to you as a physicist,
Speaker 3: but I guess for me, I'm just curious about which
Speaker 3: has more of an effect to make something transparent, right,
Speaker 3: Because you know, people always talk about like material like
Speaker 3: our skin, our bodies, they're made out of atoms. But
Speaker 3: the atoms are really sort of far apart and the
Speaker 3: nucleus is really far apart from the electrons, So there's
Speaker 3: a lot of sort of like supposedly empty space, even
Speaker 3: within solid matter like ourselves. I'm just wondering, like, you know,
Speaker 3: there's it seems like there's a lot of space for
Speaker 3: a light to squeeze through or to pass by without
Speaker 3: even knowing there are other things there.
Speaker 1: That's an interesting point, and I think a lot of
Speaker 1: people have maybe the wrong mental picture of what an
Speaker 1: atom sort of looks like to a photon. You know,
Speaker 1: even if you have a grid of atoms, like a
Speaker 1: sheet of atoms making them material, you might imagine that
Speaker 1: it's mostly empty space, as you said. And it's true
Speaker 1: that the nucleus is small compared to the distances between
Speaker 1: the atoms, right, So you have this like grid of atoms,
Speaker 1: and there is a lot of space between the nuclei.
Speaker 1: But that space is not empty, right. That space is
Speaker 1: filled with electrons and electronic fields and forces that are
Speaker 1: holding the nucleus and the electrons together. So the electrons
Speaker 1: are in these buzzy clouds all around the nucleus, and
Speaker 1: the electrons weave the atoms together, right, So it's not
Speaker 1: like there's space between the atoms. The atoms are held together,
Speaker 1: they're in these bonds that tie them together into a
Speaker 1: big grid, into a big lattice, and the electrons can
Speaker 1: sometimes slide back and forth and move between the atoms,
Speaker 1: so there isn't a lot of space between the atoms.
Speaker 1: And then inside the atom, all that space you imagine
Speaker 1: might be empty is really filled with electrons. So from
Speaker 1: the point of view of a photon hitting like a
Speaker 1: sheet of material, a sheet of paper or a sheet
Speaker 1: of glass or whatever, it really is hitting a wall
Speaker 1: of electrons that it can't find a way around unless
Speaker 1: you like physically punch holes in it.
Speaker 3: You just made me wonder, like when you have a material,
Speaker 3: how close together are the electrons shells between atoms, Like
Speaker 3: are are they bumping up against each other or is
Speaker 3: there a certain amount of space between.
Speaker 1: It depends a little bit on the kind of atom
Speaker 1: you have. But atoms tend to bond with their outermost electrons,
Speaker 1: and we'll dig into that if you let me talk
Speaker 1: about how these electrons move between the atoms. But you
Speaker 1: really are tying the outer levels of the electron orbitals
Speaker 1: together and then informing these common electron energy levels. So
Speaker 1: these atoms are sharing electrons that really are woven together.
Speaker 3: I see. I think what you're saying is that to
Speaker 3: a photon a grid of electrons, it's like a solid
Speaker 3: wall almost because it's full of electron fields, and photons
Speaker 3: interact with electrons. But like, if you were something else
Speaker 3: that was not an electron, you would maybe see a
Speaker 3: lot of empty space. But because you are a photon,
Speaker 3: then you're walking into a solid wall of electron fuzziness.
Speaker 1: Yeah, if you're a neutrino, for example, and you don't
Speaker 1: feel electric charge, then the fact that there are electromagnetic
Speaker 1: fields all through these materials is relevant to you, and
Speaker 1: you pass right through it. So like even a block
Speaker 1: of lead is basically transparent to a neutrino because it
Speaker 1: doesn't interact with the stuff. So transparency comes down to
Speaker 1: whether you interact with the material there, not really whether
Speaker 1: there are holes there. So photons they can either pass
Speaker 1: through a material or they can interact with it, and
Speaker 1: that depends on the atomic structure and the energy levels
Speaker 1: that the electrons have and whether or not they interact
Speaker 1: with that photon. Even though a photon sees a wall
Speaker 1: of material, a buzzing blob of electrons in front of it,
Speaker 1: it can still sometimes pass right through without interacting, and
Speaker 1: that's what transparency is.
Speaker 3: Yeah, I guess it can be transparent not just to light,
Speaker 3: but to other things. Right, Like, isn't there a famous
Speaker 3: like gold foil experiment that kind of help people figure
Speaker 3: out the structure of the atom.
Speaker 1: Yeah, that's right. Ruththerford shot alpha particles, which are helium
Speaker 1: nuclei and a very thin tissue of gold, and he
Speaker 1: expected it to mostly just pass through, occasionally get distorted
Speaker 1: a little bit by forces. What he found was that occasionally,
Speaker 1: like one in eight thousand times, the alpha particle the
Speaker 1: helium nucleus bounced right back. Was told him that it
Speaker 1: was interacting with something hard at the core. So that's
Speaker 1: what told him that the sheet of gold actually was
Speaker 1: made out of a grid of hard nuclei, that most
Speaker 1: of the gold was transparent to the alpha particle, but
Speaker 1: that occasional little dots of it we're.
Speaker 3: Not because I guess the helium nuclei would only interact
Speaker 3: with the nuclei of the gold. Right to the helium particle.
Speaker 3: It did look like a screen because there is a
Speaker 3: lot of empty space to a helium atom in between
Speaker 3: the gold atoms.
Speaker 1: Yeah, that's mostly true. The nuance there is that the
Speaker 1: helium does interact with the electrons. It's just that the
Speaker 1: electrons don't have the mass or the kinetic energy to
Speaker 1: bounce it back. They can only slightly change its direction.
Speaker 1: So if the helium goes through the gold and only
Speaker 1: interacts with the electrons, it gets a little bit of
Speaker 1: change of direction, whereas if it hits one of the nuclei,
Speaker 1: those have the mass like push it all the way back.
Speaker 1: So the sort of two different kinds of interaction the
Speaker 1: helium can do, one where it punches through and the
Speaker 1: other one where it bounces back.
Speaker 3: All right, So you're saying, let's maybe focus on light transparency.
Speaker 3: And we know that light interacts with electrons, and so
Speaker 3: material stuff to a photon looks pretty dense and so
Speaker 3: you can't sort of just go through it without possibly
Speaker 3: interacting with things. But you're saying, maybe the key transparency
Speaker 3: is it sometimes like doesn't interact with the thing that's there.
Speaker 1: That's right. Even if you have a bunch of electrons,
Speaker 1: photons aren't necessarily able to interact with them. The photon
Speaker 1: has to have the right amount of energy so the
Speaker 1: electron can accept it. Can absorb it. If photon has
Speaker 1: the wrong energy, it'll pass right through without interacting without electron.
Speaker 1: And that all comes down to how the electrons are
Speaker 1: confined in the material. I mean a random free electron
Speaker 1: in space. So you just have like an electron flying
Speaker 1: through the universe and a photon hits it. It's going
Speaker 1: to absorb that photon no problem, because electrons flying through
Speaker 1: space can have any energy, there's no restriction, can have
Speaker 1: any arbitrary amount of energy, so it will almost always
Speaker 1: absorb that photon. But an electron around an atom has
Speaker 1: different rules. Because of the quantum mechanics. It can only
Speaker 1: exist on like a ladder of energy levels, so it
Speaker 1: can only absorb photons that move it up one or
Speaker 1: two or ten steps on that ladder. It can't move
Speaker 1: up like one and a half steps or two point
Speaker 1: seven steps, so that limits the kinds of poltons that
Speaker 1: an electron can absorb.
Speaker 3: Interesting, it's like the electrons are stuck and they don't
Speaker 3: want to move from where they are, And so let's
Speaker 3: get a little bit deeper into that, and then also
Speaker 3: talk about what happens when something is transparent. So we'll
Speaker 3: talk about that, but first let's take a quick break.
Speaker 3: All right, we're talking about transparency here today and what
Speaker 3: makes things see through. Daniel, you mentioned that it's kind
Speaker 3: of about how light can go into material, see the material,
Speaker 3: be near the material, but not interact with it. And
Speaker 3: you say it has something to do with the energy
Speaker 3: levels of the electron, because you said that an electron
Speaker 3: floating out in space will always absorb an electron no
Speaker 3: matter what does it have to be like flying near it,
Speaker 3: does it have to hit the electron right in the middle,
Speaker 3: or does it interact when it's flying nearby? How does
Speaker 3: that free space case work?
Speaker 1: If you zap an electron with a photon and that
Speaker 1: electron is out in free space, it means that there
Speaker 1: are no rules that govern the energy that the electrons
Speaker 1: can have. I think something that's really cool and not
Speaker 1: like widely enough understood, is where quantization comes from, Like
Speaker 1: why electrons in materials have energy levels. Where these quantum
Speaker 1: energy levels come from? And it really comes from boundary conditions.
Speaker 1: It comes from forcing the electron to live within a
Speaker 1: certain location, like putting it in a box, so that
Speaker 1: electron out in empty space, it can be here, it
Speaker 1: can be there, it can have any location. It can
Speaker 1: have any momentum, and so it's free to absorb a
Speaker 1: photon of any energy. There's a probability for an electron
Speaker 1: and a photon to interact. There's still a chance, of course,
Speaker 1: that a photon will not interact with an electron. It
Speaker 1: depends on the strength of the force essentially that controls
Speaker 1: like the probability for these things to happen. But for
Speaker 1: today's conversation, we can imagine that it basically just always happens.
Speaker 1: You zap an electron with a photon. It doesn't matter
Speaker 1: what the energy of that photon is. The electron can
Speaker 1: accept it because it could have a higher momentum of
Speaker 1: any value. That's not true for electrons around an atom.
Speaker 1: It can only have energies of certain values because it's
Speaker 1: confined into the box of the atom.
Speaker 3: Now, within that free space electron, when that light hits it,
Speaker 3: what happens to that electron It gets faster or it
Speaker 3: gets hotter, and does it start spinning faster? What happens
Speaker 3: when you use zap an electron in space would.
Speaker 1: Like, well, it absorbs the momentum of that photon because
Speaker 1: of conservation momentum, and now it carries that momentum as well.
Speaker 1: And so if the photon was moving in the same
Speaker 1: direction as the electron, then it gives it a zip.
Speaker 1: It's going faster. If the photon was moving in the
Speaker 1: opposite direction, the electron hits the photon sort of head on,
Speaker 1: then it gets slowed down. Right. We talked about like
Speaker 1: laser cooling once on the podcast. You can use lasers
Speaker 1: to slow things down. Also if you zap things in
Speaker 1: the right direction.
Speaker 3: Yeah, that was pretty cool. And so it's sort of
Speaker 3: like a billier ball. I guess, like if you have
Speaker 3: an electron other and you hit it with a photon,
Speaker 3: it basically happens like it does when you hit a
Speaker 3: billier ball, right with the white ball.
Speaker 1: Yeah, the quantum mechanics comes in with the probability there's
Speaker 1: like a chance that the interaction will happen, a chance
Speaker 1: that it won't. But for today's conversation, we can think
Speaker 1: of it like a billiard ball. You're giving it a
Speaker 1: push and it's absorbing that energy. And because a free
Speaker 1: electron one out in the middle of space can have
Speaker 1: any energy, quantum mechanics is fine with that. Now you
Speaker 1: take that same electron, you say, okay, you're now in
Speaker 1: orbit around a hydrogen atom. Well, you still have to
Speaker 1: obey the rules of quantum mechanics, and in this case,
Speaker 1: quantum mechanics says are there's only certain solutions to the
Speaker 1: math here. Only certain energies of the electron make the
Speaker 1: math work. The wave function of the electron has to
Speaker 1: satisfy some conditions, and that's only true for a certain
Speaker 1: values of the electron energy. So you can't have an
Speaker 1: electron with an arbitrary energy around a proton. There's a
Speaker 1: ladder of values there, and that determines whether the electron
Speaker 1: can absorb the energy of a passing photon.
Speaker 3: I guess it's sort of like, you know, the Earth
Speaker 3: is going around the Sun in an orbit, but the
Speaker 3: Earth is it's not restricted to what that orbit can be.
Speaker 3: Like if a meteor hits Earth within a force, it
Speaker 3: is going to speed us up or slow us down,
Speaker 3: and it's going to change the path of our orbit.
Speaker 3: But you're saying sort of like in an electron around
Speaker 3: an atom, it's not like it can be in any orbit.
Speaker 3: It can only be in like certain slots of that orbit.
Speaker 3: Like it can orbit here or over there, or in
Speaker 3: this circle or in that circle. I know it's not
Speaker 3: really a circle, but it's sort of that. I mean,
Speaker 3: it can only circle around the nuclei or a certain grooves, right.
Speaker 1: Yeah, that's a great contrast because the example of Earth
Speaker 1: is a classical example. There's no quantum mechanics there. We're
Speaker 1: talking about gravity, which is a classical theory, and there's
Speaker 1: an infinite number of possible solutions for an orbit. If
Speaker 1: you pick a radius for the Earth's orbit, I can
Speaker 1: tell you exactly what velocity it has to have in
Speaker 1: order to have that orbit. So there's an infinite number
Speaker 1: of possible orbits there. In the case of the electrons,
Speaker 1: it's really very different mathematics that determines whether the electron
Speaker 1: can be in a particular state or not. You said,
Speaker 1: it's not really in an orbit, it's in a quantum state,
Speaker 1: which means it's satisfying a different equation. In this case,
Speaker 1: it's Schrodinger's equation, which is a quantum equation of the
Speaker 1: wave function, and that wave function has periodicity to it,
Speaker 1: so the wave function basically has to wrap itself around
Speaker 1: the atom in a way that builds upon itself. It
Speaker 1: doesn't cancel itself out, So you can fit in like
Speaker 1: an integer number of half wavelengths of this wave function,
Speaker 1: so that things like support each other. You get like
Speaker 1: a standing wave solution effectively instead of things like canceling
Speaker 1: themselves out. Just the same way that like on a
Speaker 1: guitar string, you can have a certain number of modes
Speaker 1: of a guitar string, right, it can oscillate the whole string,
Speaker 1: or you can have a node in the middle so
Speaker 1: both halves are oscillating, or you can have two nodes,
Speaker 1: or you get like three little oscillating pieces. In the
Speaker 1: same way, the electron has to satisfy a wave equation,
Speaker 1: not a gravitational equation, and that's where the energy levels
Speaker 1: come from comes from confining it to being around the atom,
Speaker 1: which changes the solutions to the equation.
Speaker 3: Okay, so now I have an electron orbiting around a nuclei.
Speaker 3: It's a wave function, it's a quantum object. It's sort
Speaker 3: of like snaps into a certain wave shape around the
Speaker 3: nuclei and you're saying that a photon hits it, and
Speaker 3: the electron is like nope, I like where I am now?
Speaker 3: No thanks, or that's not enough to get me to
Speaker 3: the next step in the ladder. I'm just gonna totally
Speaker 3: ignore you. Is that what's happening.
Speaker 1: That's exactly what's happening. If a photon comes along and
Speaker 1: it has enough energy to bump the electron to the
Speaker 1: next stage, it gets absorbed. If it has too much
Speaker 1: energy to get the electron to the next stage and
Speaker 1: not enough to get it like two steps up, then
Speaker 1: it gets ignored. Right, So it gets absorbed if it
Speaker 1: has the right energy to move the electron up one
Speaker 1: or two or seven some integer number of levels, and
Speaker 1: it gets ignored. If the electron would not have a
Speaker 1: solution anymore if it absorbed this photon, then it just
Speaker 1: doesn't happen. Wait.
Speaker 3: Wait, wait, So, like if an electron is going around
Speaker 3: a nuclei, it's in a cloud, and it gets one
Speaker 3: and a half as much energy from a photon that
Speaker 3: it needs to get to the next level, it's not
Speaker 3: going to take that one and then throw away the remainder.
Speaker 3: It's just totally gonna ignore the whole thing.
Speaker 1: Just totally gonna ignore the whole thing. It can absorb
Speaker 1: something that has two steps and then emit one, right,
Speaker 1: or it can absorb something that has like seven and
Speaker 1: emit four photons, But it has to be on that ladder.
Speaker 3: How exact does it need to be, like exactly to
Speaker 3: the one infinite decimal, you know what I mean? Like,
Speaker 3: where are the chances that the photon will have the
Speaker 3: exact amount of energy needs or does it just need
Speaker 3: to be around the same energy.
Speaker 1: Well, there's always uncertainty in quantum mechanics, right, So every
Speaker 1: photon has an uncertain amount of energy. You can never
Speaker 1: measure it precisely.
Speaker 3: Is that true? Really? I thought they had like specific frequencies.
Speaker 1: Well, a photon is created by a quantum process, which
Speaker 1: usually means that there is some uncertainty there. Right, there's
Speaker 1: always a little bit of fuzz in all of these
Speaker 1: processes which allow these things to overlap. The energy levels
Speaker 1: that we're talking about come from a simplified model of
Speaker 1: the nucleus, right, and in reality these things are a
Speaker 1: little bit fuzzier.
Speaker 7: Right.
Speaker 1: The physics is a little bit more complicated. There's other interaction,
Speaker 1: so there's always a little bit of fuzz on these
Speaker 1: energy levels. And the atom that produced the photon on
Speaker 1: the other side of the universe or whatever, may have
Speaker 1: produced it at a certain energy level or a little
Speaker 1: bit higher, a little bit lower, So there's enough fuzz
Speaker 1: and quantum mechanics to mean that there's a non zero
Speaker 1: probability for the photon to have the right energy to
Speaker 1: be absorbed by the electron.
Speaker 3: Hmmm. I guess when they interact, then the wave functions
Speaker 3: collapse and then you figure out if it has the
Speaker 3: right amount. But it seems very unlikely they would have
Speaker 3: the exact same amount.
Speaker 1: Yeah, that's where the fuzz comes in. So you get
Speaker 1: a little bit of width to these things, so you
Speaker 1: have a probability for them to overlap. It's not like
Speaker 1: you're throwing a dart in an infinitely sized board and
Speaker 1: having to hit exactly the right spot.
Speaker 3: I see. It's like you have a fuzzy dart and
Speaker 3: the target is fuzzy too, and as long as you
Speaker 3: sort of get it in the rhine ballpark, then it's
Speaker 3: going to knock that electron or not.
Speaker 1: Yeah, exactly. And that explains a lot of atomic physics, right.
Speaker 1: That explains why certain gases look certain colors. That explains
Speaker 1: why when you have a fire you might get like
Speaker 1: green or blue flashes in it. Or if you did
Speaker 1: that experiment in high school chemistry where you put copper
Speaker 1: in your bunsen and it glows green, explains a lot
Speaker 1: of atomic physics because different kinds of materials have different
Speaker 1: energy levels, so they glow with different frequency photons and
Speaker 1: they can absorb different frequency photons. And that's really cool
Speaker 1: because it means we can tell what's in distant stars
Speaker 1: because we can look at the spectrum of energy that
Speaker 1: they emit. We can say, oh, look, these things are
Speaker 1: emitting photons from the energy level that only comes from copper,
Speaker 1: so we can tell there's copper in that star. Sometimes
Speaker 1: these things appear as spikes in the spectrum. Sometimes they
Speaker 1: appear as dips in the spectrum because like the atmosphere
Speaker 1: of the star is absorbing those photons. But the point
Speaker 1: is that there are energy levels to the atom, and
Speaker 1: those determine whether the photon can interact with the electrons
Speaker 1: around the atom or whether it gets ignored.
Speaker 3: All right, So then we're talking about transparency. And so
Speaker 3: if I shoot a photon at an atom, it's gonna
Speaker 3: get up to the electron cloud there and it's going
Speaker 3: to be like, no, I'm not the right energy. I'm
Speaker 3: just going to keep going. Or is it the case
Speaker 3: that they do interact? But then the end result is
Speaker 3: the same and it just spits out a photon of
Speaker 3: the same energy in the same direction.
Speaker 1: Now, if they're the wrong energy, they just do not interact.
Speaker 1: If they're the right energy, it gets absorbed and then
Speaker 1: it can get re emitted. And that's a whole complicated
Speaker 1: phenomena about reflection and refraction and all sorts of stuff.
Speaker 1: In this case, for transparency, it's more about whether there's
Speaker 1: an interaction. If it has the wrong energy levels, it
Speaker 1: just doesn't interact. But that's the case for a single atom,
Speaker 1: which is not really what's going on when you're looking
Speaker 1: at light going through glass or when you're wondering why
Speaker 1: light doesn't go through metal. It's much more complicated because
Speaker 1: now you're packing a lot of atoms together, and so
Speaker 1: the rules about what happens to those electrons now change.
Speaker 3: M let's dig into that. What's going on there.
Speaker 1: So remember the picture we were talking about earlier. When
Speaker 1: a photon is approaching like a sheet of metal or
Speaker 1: a sheet of iron or a sheet of marble or something,
Speaker 1: it's facing a whole wall of atoms, not individual atoms.
Speaker 1: We talked about the energy levels of an individual atom.
Speaker 1: But when you bring these things together to make a grid,
Speaker 1: then the atoms bond They're not just like near each other,
Speaker 1: they really are bonded. And if you remember your high
Speaker 1: school chemistry, that means that they are sharing electrons. Sometimes
Speaker 1: the electron will like be around one nucleus, sometimes around
Speaker 1: another nucleus. So from the point of view of the electron,
Speaker 1: what happens is that you no longer really belonging to
Speaker 1: one nucleus. Now you can think of like the whole
Speaker 1: grid of nuclei as having a bunch of energy levels
Speaker 1: for the electrons. Some of the inner electrons are trapped
Speaker 1: around nucleus, but the outer electrons can flow between them,
Speaker 1: and that creates a whole complicated set of energy levels.
Speaker 1: And instead of having these very specific ladders, now you
Speaker 1: have this like spectrum energy levels. The electrons have lots
Speaker 1: more options of the energy levels they can be at.
Speaker 3: I guess I wonder if it's sort of like, you know,
Speaker 3: we're orbiting around the Sun and we're sort of stuck
Speaker 3: in this orbit, but if another solar system came pretty close,
Speaker 3: maybe Jupiter might be like, oh, sometimes it might do
Speaker 3: like a little figure eight and sometimes leave our solar
Speaker 3: system and go take a loop around that other Sun
Speaker 3: and then come back and is that sort of what's
Speaker 3: happening to the electrons.
Speaker 1: Yeah, that's exactly what's happening to the electrons. There's lots
Speaker 1: more options for them. They don't have to just stick
Speaker 1: around one nucleus. They interact with lots of different nuclei.
Speaker 1: So that has the consequence of sort of spreading these
Speaker 1: sharper atomic orbitals and making them even fuzzier. So instead
Speaker 1: of even really thinking about energy levels now, physicists talk
Speaker 1: about the possibilities for electrons in these materials as energy bands,
Speaker 1: and you may have heard of like the valence band
Speaker 1: or the conduction band. These are like a spectra of
Speaker 1: energy levels available to the electron. Instead of being more
Speaker 1: like a ladder, they get blurred together, so there's lots
Speaker 1: of really really fine steps. It's still technically a ladder,
Speaker 1: but there's many many more steps there.
Speaker 3: Yeah, I guess it's sort of like in one atom,
Speaker 3: the electron's stuck in one particular rut or groove or orbit.
Speaker 3: That's one extreme. The electron is a free floating electron
Speaker 3: adem sit in space by itself. When atoms are sort
Speaker 3: of bonded together in the material, you're saying the electrons
Speaker 3: are sort of in the middle, like they're not quite
Speaker 3: stuck to one particular atom, but they're not quite free either,
Speaker 3: and so they have limited options. But they don't have
Speaker 3: just one option, and so there's sort of a range
Speaker 3: of photons they can absorb.
Speaker 1: Yeah, exactly. And it depends a little bit on the
Speaker 1: temperature of the object. If the object is really really
Speaker 1: cold and the electrons don't have a lot of energy,
Speaker 1: then they've all like settled down to their minimum energy
Speaker 1: and mostly they are orbiting an individual nuclei and they're
Speaker 1: mostly stuck, and so the electrons don't flow very much.
Speaker 1: If the thing is hot, then a lot of the
Speaker 1: electrons have more energy. They have enough energy to like
Speaker 1: hop from nuclei to nuclei, and so they can flow
Speaker 1: a little bit better.
Speaker 3: Wait, what so that if I heat something up or
Speaker 3: cool it down, I can make it go transparent or
Speaker 3: not transparent.
Speaker 1: No, by heating it up, you're not changing the energy
Speaker 1: levels that are available. You're just changing where the electrons are.
Speaker 1: Like instead of all being in the lowest energy levels,
Speaker 1: now they're in higher energy levels. I'm just talking about
Speaker 1: which energy levels are filled up. In some cases, the
Speaker 1: electrons are sort of stuck when this stuff is cold,
Speaker 1: the electrons fill the lower energy levels and they're more
Speaker 1: stuck to the nuclei, and when the object is warmer,
Speaker 1: they sort of jump out of those and they're freer
Speaker 1: to move around from nucleus to nucleus.
Speaker 3: Cool. Well, let's get a little bit deeper into the
Speaker 3: material and see what happens when photons of different frequencies
Speaker 3: try to go through it, and what it all means
Speaker 3: about transparency in the universe. But first, let's take another
Speaker 3: quick break. All right, we're talking about transparency, and something
Speaker 3: that comes to mind is, I don't know if you
Speaker 3: read old comic books, or you read comic books when
Speaker 3: you were a kid, there was always an ad in
Speaker 3: the bag for like X ray glasses, and I always wondered,
Speaker 3: like are those for real? Or like how can they
Speaker 3: sell something so bogus out of what's going on? I
Speaker 3: always wanted to order one, but I couldn't because I
Speaker 3: wasn't in Panama. Do you know what I'm talking about?
Speaker 3: Do you know what they were actually selling?
Speaker 1: Do know those ads? And I also wanted those and
Speaker 1: I wanted them to be real, but I also never
Speaker 1: bought them because I was pretty sure they were bogus.
Speaker 1: I mean, you can see through things with X rays,
Speaker 1: and we'll talk about why that happens, why high energy
Speaker 1: photons from X rays can pass through materials sometimes when
Speaker 1: lower energy photons can't. But those glasses can't let you
Speaker 1: see X rays, and they definitely don't generate X rays, right,
Speaker 1: You're not shooting X rays through stuff. So I'm pretty
Speaker 1: sure it was totally bogus.
Speaker 3: Or maybe not. I don't know. We can't say for sure.
Speaker 1: Well, folks out there, if you bought those X ray
Speaker 1: glasses and they did let you see through things, please
Speaker 1: write to us and let us hear.
Speaker 3: Those people are probably rich from a you know, stealing
Speaker 3: a bank vaults and things like that. All right, So
Speaker 3: we're talking about transparency. And you know, when you put
Speaker 3: a bunch of adoms together in the material, they form
Speaker 3: this kind of extended fuzzy cloud of electrons that might
Speaker 3: blocklide or not. And so whether a photon gets through
Speaker 3: that depends on its energy. If it has the energy
Speaker 3: that the electrons in that material like, then it gets
Speaker 3: it's going to get absorbed right and not go through exactly.
Speaker 1: So the basic picture is the same photon approaches this
Speaker 1: now grid of atoms and if it finds an electron
Speaker 1: that can accept its energy, if the electron can go
Speaker 1: from its current quantum state to an allowed quantum state,
Speaker 1: it will absorb that photon. But the picture of the
Speaker 1: energy levels is different from a single atom than with
Speaker 1: the grid of atoms, and the single atom you had
Speaker 1: the ladder, there was sort of sharper energy levels in
Speaker 1: the grid of atoms. Now you have these bands of
Speaker 1: energy levels, and you might think, oh, that makes it
Speaker 1: possible for the electron to absorb basically any photon. It's
Speaker 1: a little bit more complicated than that because we discovered
Speaker 1: that there are these gaps in the energy levels. It's
Speaker 1: not like any possible energy level is allowed for an
Speaker 1: electron in these materials, the way it is for an
Speaker 1: electron in free space. There are still electron energy levels
Speaker 1: that are not allowed. So there's this band of electron
Speaker 1: energies called the valiance band, where the electrons mostly hang
Speaker 1: out in a random material, and then there's a band
Speaker 1: of energies called the conduction band, where electrons can move
Speaker 1: around really freely from atom to adm them and there's
Speaker 1: sometimes a gap between them where electrons can't be and
Speaker 1: that can prevent electrons from absorbing energy of a passing photon.
Speaker 3: M you're saying, like, you can have a material that
Speaker 3: accepts or lets through lights of a certain range of frequencies,
Speaker 3: then it doesn't let them through, and then it does
Speaker 3: for a different range of frequencies, it does lead light.
Speaker 1: Through exactly just like with the atom. It can absorb
Speaker 1: some frequencies and not other frequencies for a grid of atoms.
Speaker 1: For a whole solid material, it can absorb some frequencies
Speaker 1: frequencies where it can hit the electron and jump it
Speaker 1: over this gap between the bands, and it can't absorb
Speaker 1: photons of other frequencies, photons that don't have enough energy
Speaker 1: to get the electrons from one band to another. And
Speaker 1: so different kind of materials have a different sized gap
Speaker 1: between these bands, and so in solid state physics they
Speaker 1: call this the band gap, right, the gap between the
Speaker 1: typical energy levels of the electron and the conduction band
Speaker 1: where electrons are good at like flowing, and some kind
Speaker 1: of materials like meta have a very very small band gap.
Speaker 1: The conduction band is basically right on top of the
Speaker 1: valiance band. There's basically no gap there, and so electrons
Speaker 1: are very good at absorbing photons of a huge range
Speaker 1: of energies because there's a huge spectrum there and other
Speaker 1: materials is a big gap. And in order for an
Speaker 1: electron to absorb a photon, it has to have enough energy,
Speaker 1: and lots of photons just don't have enough energy, and
Speaker 1: so the photons would pass right through the material without interacting.
Speaker 3: Now, when you're talking about light and energy, the light
Speaker 3: of a particular photon is related to its frequency, right
Speaker 3: mostly to it almost or everything to its frequency, and
Speaker 3: so you're really talking about its color, right.
Speaker 1: Yeah, exactly. The energy of a photon doesn't relate to
Speaker 1: its speed. Right. When we think about the energy of
Speaker 1: an electron, we think about its speed, but photons are
Speaker 1: all moving at the same speed. The thing that differentiates
Speaker 1: a high and low energy electron is its frequency, how
Speaker 1: fast the electromagnetic fields are wiggling. And as you say
Speaker 1: that frequency we interpret as color. The photons themselves don't
Speaker 1: have color. It's not like a photon is a red
Speaker 1: photon or a green photon just has a certain frequency
Speaker 1: when it hits our eyeballs our brains give us the
Speaker 1: experience of red or green or blue or whatever, and
Speaker 1: that's a whole philosophical question. But yeah, we associate colors
Speaker 1: with certain frequencies.
Speaker 3: Yes, but we don't talk about philosophy here. But like that,
Speaker 3: if a photon has a certain frequency, it is a
Speaker 3: red photon, right, Like to our eyes it would read
Speaker 3: as red.
Speaker 1: It would read as red. Yeah, And some photons are
Speaker 1: above the visible spectrum, and so we say they're X
Speaker 1: ray photons, or their gamma rays, or their UV photons, right,
Speaker 1: So we can give names to the different parts of
Speaker 1: the frequency spectrum. Some of them we give them colors,
Speaker 1: some of them we just give them labels. Radio waves,
Speaker 1: for example, our photons a very very long frequency, well
Speaker 1: below what we can see, even below the infrared.
Speaker 3: Right, And so that determines whether or not a materials
Speaker 3: transparent to different kinds of light. Because X rays sort
Speaker 3: of lead you see through your body and your bones, right.
Speaker 3: That's because they have a high energy and the electrons
Speaker 3: in your body can absorb them, so they sort of
Speaker 3: go through.
Speaker 1: Yeah. Really interestingly, X rays can pass through the soft
Speaker 1: tissues of your body, but they can't pass through your bones,
Speaker 1: which is why when you see an X ray, what
Speaker 1: you're looking at is basically only the bones. Because that's
Speaker 1: the thing that the X rays didn't pass through, so
Speaker 1: it passes through everything else. Your body is transparent to
Speaker 1: X rays except for your bones. That's why you can
Speaker 1: tell the difference between the bones and the not bones
Speaker 1: part on the X ray.
Speaker 3: Now is that because the I guess bones are made
Speaker 3: out of the different material than my muscles, and so
Speaker 3: my muscles don't absorb X rays what my bones do
Speaker 3: because of the you know, the bonds between the atoms exactly.
Speaker 1: It's the band gap of the material that determines whether
Speaker 1: or not you can absorb photons of a specific frequency. So,
Speaker 1: for example, in a conductor like a metal, like a
Speaker 1: sheet of steel, the band gap is really really small.
Speaker 1: It's very easy to get an electron up into that
Speaker 1: conduction band where it can low around. And that's why
Speaker 1: these things conduct electricity very easily because it's easy to
Speaker 1: have electrons that slide around in the material. So conductor
Speaker 1: like a metal, right, it's really easy to get those
Speaker 1: electrons flying around. It also means it's easy to absorb
Speaker 1: that energy. So that's why things like metals and conductors
Speaker 1: are good at conducting electricity and good at absorbing photons
Speaker 1: and bad at being see through. So that's why a
Speaker 1: sheet of metal, for example, is not.
Speaker 3: Transparent unless it's I guess a wire mesh.
Speaker 1: Yeah, exactly, unless it's a screen, which is why if
Speaker 1: you remember like Star Trek and they had like transparent
Speaker 1: aluminium in Star Trek four or whatever, I always like, well,
Speaker 1: you can't really do that. That is not something we
Speaker 1: know how to do. Although you know, far future societies
Speaker 1: maybe they figured it out.
Speaker 3: I'm not sure I'm familiar with that level of trivia
Speaker 3: for the Story Trek movies.
Speaker 5: But I.
Speaker 1: Remember they got the whales and they had to build
Speaker 1: an aquarium for the whales, and how are they going
Speaker 1: to hold all this water?
Speaker 3: I remember the whales? Yeah, right, but I guess what
Speaker 3: do you mean though, Like aluminium is not it's transparent
Speaker 3: to visible light, but it is still transparent to other
Speaker 3: kinds of light, right, like X rays sort of go
Speaker 3: through metal. No, or do metals like block all light.
Speaker 1: It's always the case that it depends on the frequency, right,
Speaker 1: and so you have to have the right frequency to
Speaker 1: match the energy levels that the object can absorb. If
Speaker 1: you have a huge amount of energy, then probably you're
Speaker 1: going to knock the electrons out of the material. Right,
Speaker 1: then we're getting into the case of like the photoelectric effect.
Speaker 1: So you zap like gamma rays against aluminum, then there's
Speaker 1: definitely going to be an interaction there, but it's going
Speaker 1: to knock the whole electron out of the material. It's
Speaker 1: not just going to push it up to some energy level.
Speaker 1: So at some point this picture breaks down.
Speaker 3: Oh what So at some point you have enough energy
Speaker 3: where the electron JA doesn't someone to say in any groove,
Speaker 3: it just flies out into space.
Speaker 1: Really, yeah, exactly, you can shine light on metal and
Speaker 1: boil off electrons if you have enough energy. It's like
Speaker 1: a highest level band and above that then electrons are
Speaker 1: just free. Again, you've like broken it out of physics jail.
Speaker 3: You get to pay two hundred bucks. Though exactly, Now,
Speaker 3: what happens on the other spectrum, Like what if a
Speaker 3: photon has too little energy like a super infrared or
Speaker 3: something like that, or radio wave as you said, that
Speaker 3: still goes through metal and other things.
Speaker 1: Right, No, radio waves do not go through metal. Right.
Speaker 1: That's why, for example, your phone call is dropped if
Speaker 1: you're in an elevator because metal is like a Faraday cage.
Speaker 1: It will block radio waves even classically, right, the electrons
Speaker 1: in the material will reorganize themselves to cancel out an
Speaker 1: electric field. But from a quantum mechanical point of view,
Speaker 1: a conductor can absorb very very low energy photons because
Speaker 1: the gap there is very small, and so it can
Speaker 1: absorb very very low energy photons.
Speaker 3: But I guess what's going on there though, Like, if
Speaker 3: it's just a single atom and I have an electron orbiting,
Speaker 3: if the forodan has very little energy, I'm going to
Speaker 3: ignore it too, are I?
Speaker 1: Yeah, absolutely you are. In the case of an individual atom,
Speaker 1: then there are photons that have too low an energy
Speaker 1: to move the electron up from level one, level two,
Speaker 1: or level seven to level eight exactly. That can happen
Speaker 1: in a solid. Now you have a whole spectrum of
Speaker 1: energy levels, and so there's lots of very very fine
Speaker 1: gradations allowed there. So materials can absorb low energy electrons
Speaker 1: because there's a very very fine mesh of energy levels.
Speaker 3: Is there a bottom limit there? Like, is there an
Speaker 3: energy for my photon for which it's even outside of
Speaker 3: the gap of material with lots of electrons.
Speaker 1: There might be a lower limit there. I mean, even
Speaker 1: conductors do have some kind of a band gap, so
Speaker 1: you might need a minimum energy to get them up
Speaker 1: from the valiance band to the conduction band, and there
Speaker 1: might even be a limit within those bands a minimum energy,
Speaker 1: so the other might be a limit. Very very low
Speaker 1: energy photons could be ignored even by conductors. But the
Speaker 1: other side of the coin are materials like insulators. Take glass,
Speaker 1: for example. Glass is not a conductor because it has
Speaker 1: a large gap between these energy levels. So mostly electrons
Speaker 1: in the glass are not free to move around. They're
Speaker 1: mostly stuck to the atom that they are around, and
Speaker 1: there's a band gap there. If you want to push
Speaker 1: an electron up to the next energy level, there's like
Speaker 1: a big gap between the energy levels that's normally in
Speaker 1: and the first one that's available. It's like a few
Speaker 1: electron bolts, and so photons that hit glass in the
Speaker 1: visible spectrum mostly do not have enough energy to get
Speaker 1: the electron up to the conduction band. And so that's
Speaker 1: why visible light photons do pass through glass. They pass
Speaker 1: right through this whole grid of atoms and all those electrons,
Speaker 1: but they don't have enough energy to move the electrons
Speaker 1: up to the next band, and so they're ignored and
Speaker 1: they pass right through.
Speaker 3: You mean like a material like glass. It's like the
Speaker 3: atoms are basically it's just a bunch of individual atoms
Speaker 3: hanging out together. They're not sharing a lot of electrons,
Speaker 3: which is what you need to make a good conductor.
Speaker 3: They're mostly just doing what they would do normally on
Speaker 3: their own, and so you have a very limited number
Speaker 3: of frequencies that it blocks.
Speaker 1: The picture is a little bit more complicated. I mean
Speaker 1: the glass atoms still do interact with each other, so
Speaker 1: they do this band of energy levels for the electrons
Speaker 1: because they are bonded together, right, I mean, glass is
Speaker 1: not a crystal, but still there are bonds between the
Speaker 1: atoms they are interacting, so there is a spectrum of
Speaker 1: energy levels the electrons can be in For a glass
Speaker 1: that's not just like an atom. It's not just like
Speaker 1: a sharp layer, but it's mostly a full band of electrons.
Speaker 1: But that band is mostly filled. Then the electrons can't
Speaker 1: really go anywhere. It's like if you're on a plane,
Speaker 1: every seat is taken, then you can't like move from
Speaker 1: seat to seat, and the next energy level above that
Speaker 1: is kind of high. Right, you can't like get to
Speaker 1: first class. You need a lot of energy to get
Speaker 1: to first class in a glass, and so everybody's basically
Speaker 1: stuck in their seat in coach, and the electrons can't
Speaker 1: really absorb little amounts of energy. They need a lot
Speaker 1: of energy to get promoted up to first class, which
Speaker 1: is the next band of energy levels. In a glass
Speaker 1: and a conductor, that band is much much lower, so
Speaker 1: it doesn't take as much energy to get up there,
Speaker 1: and glass the band is really large, it's really hard
Speaker 1: to get promoted up to the next set of energy levels.
Speaker 3: I think what you're saying is that glass is transparent
Speaker 3: for a wider range of frequencies of light, which just
Speaker 3: happened to be in our visible spectrum. But glass is opaque.
Speaker 3: It is not transparent to certain frequencies of light.
Speaker 1: That's exactly right. For example ultraviolet, right, ultraviolet is light
Speaker 1: with higher energy. You can't see ultraviolet light. It's the
Speaker 1: kind that's going to give you a sunburn or can
Speaker 1: hurt your eyeballs. But it has more energy, and it
Speaker 1: has enough energy to bump one of these electrons up
Speaker 1: over this gap into the conduction band, and so glass
Speaker 1: can absorb uv photons. That's why you don't get a
Speaker 1: sunburn if you're sunbathing through glass, glass is like sunscreen.
Speaker 3: Wait what I can just put a glass over me
Speaker 3: and I'm going to give you sunburn? Is that a
Speaker 3: solid medical advice there?
Speaker 1: That is not solid medical advice?
Speaker 3: And just to be transparent, we're non medical.
Speaker 1: Doctors, right, that's right. But it does block some of
Speaker 1: the UV, so it would reduce your sunburn. It still
Speaker 1: totally advise you to wear sunscreen, but glass is not transparent,
Speaker 1: and to UV, the same way is to visible light,
Speaker 1: absorbs a lot more of the UV.
Speaker 3: Is that kind of what's going on with sunscreens? Like lotion?
Speaker 5: Right?
Speaker 7: Right?
Speaker 3: It has materials that absorb UV.
Speaker 1: Right, Yeah, exactly, your sunscreen is just opaque to UV.
Speaker 1: It has stuff in it that could accept those photons
Speaker 1: and absorb it rather than letting those UV photons pass
Speaker 1: into your body and then cause damage.
Speaker 3: Okay, Now, what makes a piece of glass like a
Speaker 3: piece of red glass or a piece of blue glass?
Speaker 1: So that often is because of doping. You like change
Speaker 1: the energy levels of the glass by adding impurities, and
Speaker 1: so these other molecules change the band gap, so make
Speaker 1: it possible for different kinds of photons to be absorbed.
Speaker 3: And so I guess you sort of narrow the band gap, right,
Speaker 3: like regular glass wide range of photon frequencies that it
Speaker 3: lets through, but like blue glass I imagine, has a
Speaker 3: narrow gap where it only lets through light that is
Speaker 3: bluish for example.
Speaker 1: That's right. So sometimes people add like aluminium oxide the glass,
Speaker 1: and that makes glass pink or red because it absorbs
Speaker 1: the green and the blue photons. And so I actually
Speaker 1: got an email from a listener, Matt Cleveland, who says,
Speaker 1: what is it about the photons of sunlight that costs
Speaker 1: some objects to fade and lose their color? What is
Speaker 1: it that's breaking down? Why do some objects lose their
Speaker 1: color from this interaction with the sun's photons and others
Speaker 1: do not?
Speaker 3: That's an interesting question. Yeah, Like if you leave your
Speaker 3: T shirt out in the sun, it's going to get faded, right,
Speaker 3: It's going to get bleached. That's kind of why your
Speaker 3: hair also gets bleached. A little bit of view. Stay
Speaker 3: out in the sun a lot.
Speaker 1: Yeah, exactly, And that's mostly the UV light, Right. These
Speaker 1: chemicals absorb UV light, and the UV light has a
Speaker 1: lot of energy, so sometimes it breaks down those chemicals. Right,
Speaker 1: we talked about like photons hitting electrons and banging them
Speaker 1: out of materials well. UV light. Sometimes these atoms can
Speaker 1: absorb it, but it also damages the atoms the same
Speaker 1: way like can damage things in your body, and so
Speaker 1: chemicals in objects can sometimes break down when they absorb
Speaker 1: UV light. And what you'll notice is that red stuff
Speaker 1: is especially suceptible to this because they absorb more high energy,
Speaker 1: more blue and more UV photons. So things that look
Speaker 1: red are things that absorb in the blue spectrum and
Speaker 1: therefore absorb more UV light and are likely to fade
Speaker 1: more in sunlight than things that are.
Speaker 3: Blue, because if they're red, then that means they're mostly
Speaker 3: reflecting the red part, but they're absorbing the blue light.
Speaker 1: Yeah, exactly. And UV is like super blue.
Speaker 3: But sometimes materials get harder in the sun, right, Like
Speaker 3: if you leave a piece of rubber or a rubber
Speaker 3: band or your car tires, they get more brittle as
Speaker 3: they stay out in the sun longer.
Speaker 1: Yeah, that's a similar process. You're not changing their transparency,
Speaker 1: but still the UV light is changing the chemical composition
Speaker 1: because it's being absorbed and it's breaking down some of
Speaker 1: the bonds, and it's changing the chemical nature of the substance.
Speaker 3: So if you put your car inside of a glass house.
Speaker 3: Then they'll stay the same color and the tires will
Speaker 3: stay bouncy. Is that what you're saying?
Speaker 1: Yeah? Or if you smear your car and sunscreen either one.
Speaker 3: Oh, I guess you have to be careful. You know,
Speaker 3: you know what to say about cars that live in
Speaker 3: glass houses. All right, Well, I guess it's an interesting
Speaker 3: look into a very familiar thing that is all around us, right,
Speaker 3: Like the screen on your phone is made out of
Speaker 3: transparent glass, and your windows, and every time you go
Speaker 3: to the doctor or the dentists and they take X rays,
Speaker 3: it's like physics going on.
Speaker 1: Right, There is physics going on everywhere. What's amazing to
Speaker 1: me is that sometimes we can even unravel this like
Speaker 1: microphysical picture of what's happening. Do explain our everyday experience.
Speaker 1: Why things are squishy, why things are hard, why things
Speaker 1: conduct electricity, why things are see through. It all comes
Speaker 1: down to what's happening at the atomic level or the
Speaker 1: subatomic level, And incredibly, that's a story we can sometimes
Speaker 1: understand and even explain to you.
Speaker 3: Yes, it's almost like the universe is transparent to science,
Speaker 3: or it's like scientists have X ray glasses.
Speaker 1: Or maybe the UV photons of the universe are just
Speaker 1: frying our brains.
Speaker 3: Because your skull is made at a glass. What's going
Speaker 3: on there?
Speaker 1: I'm going to go put sunscreen on my brain, yeah.
Speaker 3: Or a hat, you know. They physics have invented hats
Speaker 3: also which helps with sun damage.
Speaker 1: Quantum hats. We should sell those all right.
Speaker 3: Well, we hope you enjoyed that. Thanks for joining us,
Speaker 3: See you next time.
Speaker 1: Thanks for listening, and remember that. Daniel and Jorge Explain
Speaker 1: the Universe is a production of iHeartRadio. For more podcasts
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