1 00:00:01,920 --> 00:00:07,080 Speaker 1: Welcome to Brainstuff, a production of iHeart Radio. Hey, brain Stuff. 2 00:00:07,120 --> 00:00:11,080 Speaker 1: Lauren bogelbam here. It seems like a simple question, but 3 00:00:11,160 --> 00:00:13,920 Speaker 1: it took many centuries and a lot of smart people, 4 00:00:14,080 --> 00:00:18,400 Speaker 1: including Aristotle, Isaac Newton, Thomas Young, James Clerk, Maxwell, and 5 00:00:18,440 --> 00:00:21,680 Speaker 1: Herman von Helmholtz to puzzle out the answer to why 6 00:00:21,920 --> 00:00:26,040 Speaker 1: is the sky blue? That's because the solution encompasses so 7 00:00:26,120 --> 00:00:29,440 Speaker 1: many components, the colors in sunlight, the angle at which 8 00:00:29,440 --> 00:00:33,000 Speaker 1: solar illumination travels through the atmosphere, the size of airborne 9 00:00:33,040 --> 00:00:37,640 Speaker 1: particles and atmospheric molecules, and the way our eyes perceive color. 10 00:00:39,200 --> 00:00:41,440 Speaker 1: Let's take the sky out of the equation for a 11 00:00:41,479 --> 00:00:45,240 Speaker 1: moment and begin by looking at color from a physics standpoint. 12 00:00:45,360 --> 00:00:48,559 Speaker 1: Color refers to the wavelengths of visible light leaving an 13 00:00:48,560 --> 00:00:51,839 Speaker 1: object and striking a sensor such as a human eye. 14 00:00:52,400 --> 00:00:56,320 Speaker 1: These wavelengths might be reflected or scattered from an external source, 15 00:00:56,640 --> 00:01:00,760 Speaker 1: or they might emanate from the object itself. The color 16 00:01:00,920 --> 00:01:04,120 Speaker 1: of an object changes depending on the colors contained in 17 00:01:04,120 --> 00:01:08,479 Speaker 1: the light source. For example, red paint, when viewed under 18 00:01:08,520 --> 00:01:13,120 Speaker 1: blue light, looks black. Isaac Newton demonstrated with the prism 19 00:01:13,360 --> 00:01:16,120 Speaker 1: that the white light of the sun contains all colors 20 00:01:16,120 --> 00:01:20,000 Speaker 1: of the visible spectrum, so all colors are possible in sunlight. 21 00:01:21,319 --> 00:01:24,000 Speaker 1: In school, you may have learned that, for example, a 22 00:01:24,080 --> 00:01:28,399 Speaker 1: banana appears yellow because it reflects yellow light and absorbs 23 00:01:28,440 --> 00:01:33,200 Speaker 1: all other wavelengths. This isn't quite accurate, though, a banana 24 00:01:33,280 --> 00:01:36,000 Speaker 1: scatters as much orange and red as it does yellow, 25 00:01:36,440 --> 00:01:38,720 Speaker 1: and it scatters all of the colors of the visible 26 00:01:38,800 --> 00:01:42,240 Speaker 1: range to some degree or another. The real reason it 27 00:01:42,240 --> 00:01:46,360 Speaker 1: looks yellow relates to how our eyes sense light. Before 28 00:01:46,400 --> 00:01:48,840 Speaker 1: we get into that, however, let's look at what color 29 00:01:49,040 --> 00:01:54,920 Speaker 1: the sky actually is. Like bananas, atoms, molecules, and particles 30 00:01:54,920 --> 00:01:58,880 Speaker 1: in the atmosphere absorb and scatter light. If they didn't, 31 00:01:59,120 --> 00:02:01,760 Speaker 1: or if the Earth had no atmosphere, we would perceive 32 00:02:01,840 --> 00:02:04,400 Speaker 1: the Sun as a very bright star among others in 33 00:02:04,440 --> 00:02:08,040 Speaker 1: a sky of perpetual night. Not all wavelengths in the 34 00:02:08,080 --> 00:02:12,679 Speaker 1: visible light spectrum scatter equally. However, shorter and more energetic 35 00:02:12,720 --> 00:02:16,600 Speaker 1: wavelengths towards the violet end of the spectrum scatter better 36 00:02:16,880 --> 00:02:21,560 Speaker 1: than those towards the longer, less energetic red end. This 37 00:02:21,639 --> 00:02:24,839 Speaker 1: tendency is due in part to their higher energy, which 38 00:02:24,840 --> 00:02:28,120 Speaker 1: allows them to ping pong around more and in part 39 00:02:28,240 --> 00:02:30,880 Speaker 1: to the geometry of the particles that they interact with 40 00:02:31,040 --> 00:02:35,919 Speaker 1: in the atmosphere. In eighteen seventy one, Lord Rayleigh derived 41 00:02:35,919 --> 00:02:39,880 Speaker 1: a formula describing a subset of these interactions in which 42 00:02:39,919 --> 00:02:43,520 Speaker 1: atmospheric particles are much smaller than the wavelengths of the 43 00:02:43,600 --> 00:02:47,919 Speaker 1: radiation that are striking them. The Rayleigh scattering models showed 44 00:02:47,919 --> 00:02:51,519 Speaker 1: that in such systems, the intensity of scattered light is 45 00:02:51,560 --> 00:02:56,640 Speaker 1: inversely proportional to the fourth power of light's wavelength, which 46 00:02:56,840 --> 00:02:59,440 Speaker 1: is a really Matthew way of saying that shorter wavelengths 47 00:02:59,440 --> 00:03:02,040 Speaker 1: of light like blue and violet, scatter a lot more 48 00:03:02,120 --> 00:03:05,120 Speaker 1: than longer redder ones when the particles that they hit, 49 00:03:05,320 --> 00:03:10,120 Speaker 1: such as oxygen and nitrogen molecules, are relatively small. Under 50 00:03:10,120 --> 00:03:14,080 Speaker 1: these conditions, scattered light also tends to disperse equally in 51 00:03:14,160 --> 00:03:17,880 Speaker 1: all directions, which is why the sky appears so saturated 52 00:03:18,000 --> 00:03:21,360 Speaker 1: with color. If we were foolish enough to look directly 53 00:03:21,400 --> 00:03:25,160 Speaker 1: at the Sun, we would see all wavelengths because light 54 00:03:25,240 --> 00:03:28,600 Speaker 1: would be reaching our eyes directly. That's why the Sun 55 00:03:28,720 --> 00:03:32,120 Speaker 1: and the area around it look white. When we look 56 00:03:32,160 --> 00:03:35,440 Speaker 1: away from the Sun at the cloudless sky, we see 57 00:03:35,520 --> 00:03:40,520 Speaker 1: light mostly from shorter scattered wavelengths like violet, indigo, and blue. 58 00:03:42,080 --> 00:03:47,600 Speaker 1: So why doesn't the sky appear violet instead of light blue? Here? 59 00:03:47,640 --> 00:03:52,040 Speaker 1: The eyes have it. Your peepers perceive color using structures 60 00:03:52,040 --> 00:03:56,480 Speaker 1: called cones. Your retinas bristle with about five million cones, 61 00:03:56,560 --> 00:03:59,600 Speaker 1: each a made up of three types that specialize in 62 00:03:59,680 --> 00:04:03,400 Speaker 1: seeing different colors. Although each kind of cone is most 63 00:04:03,480 --> 00:04:06,880 Speaker 1: sensitive to certain peak wavelengths, the ranges of those cone 64 00:04:06,880 --> 00:04:11,080 Speaker 1: types overlap. As a result, different wavelengths of light and 65 00:04:11,280 --> 00:04:14,800 Speaker 1: combinations of different wavelengths can be detected as the same color. 66 00:04:16,160 --> 00:04:19,960 Speaker 1: Unlike our auditory senses, which can recognize individual instruments in 67 00:04:20,000 --> 00:04:24,279 Speaker 1: an orchestra, our eyes and brains interpret certain combinations of 68 00:04:24,279 --> 00:04:29,200 Speaker 1: wavelengths as a single, discreet color. Our visual sense interprets 69 00:04:29,279 --> 00:04:31,880 Speaker 1: the blue violet light of the sky as a mixture 70 00:04:31,960 --> 00:04:35,279 Speaker 1: of blue and white light, and that is why the 71 00:04:35,320 --> 00:04:43,480 Speaker 1: sky is light blue. Today's episode was written by Nicholas 72 00:04:43,520 --> 00:04:45,920 Speaker 1: Garbis and produced by Tyler Clang. For more on this 73 00:04:46,000 --> 00:04:48,880 Speaker 1: and lots of other curious topics, visit how stuffworks dot com. 74 00:04:49,000 --> 00:04:51,280 Speaker 1: Brain Stuff is production of I Heart Radio. For more 75 00:04:51,279 --> 00:04:53,760 Speaker 1: podcasts from my heart Radio, visit the heart Radio app. 76 00:04:53,839 --> 00:04:56,480 Speaker 1: Apple podcasts, or wherever you listen to your favorite shows.