WEBVTT - Why Is the Sky Blue?

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<v Speaker 1>Welcome to Brainstuff, a production of iHeart Radio. Hey, brain Stuff.

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<v Speaker 1>Lauren bogelbam here. It seems like a simple question, but

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<v Speaker 1>it took many centuries and a lot of smart people,

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<v Speaker 1>including Aristotle, Isaac Newton, Thomas Young, James Clerk, Maxwell, and

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<v Speaker 1>Herman von Helmholtz to puzzle out the answer to why

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<v Speaker 1>is the sky blue? That's because the solution encompasses so

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<v Speaker 1>many components, the colors in sunlight, the angle at which

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<v Speaker 1>solar illumination travels through the atmosphere, the size of airborne

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<v Speaker 1>particles and atmospheric molecules, and the way our eyes perceive color.

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<v Speaker 1>Let's take the sky out of the equation for a

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<v Speaker 1>moment and begin by looking at color from a physics standpoint.

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<v Speaker 1>Color refers to the wavelengths of visible light leaving an

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<v Speaker 1>object and striking a sensor such as a human eye.

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<v Speaker 1>These wavelengths might be reflected or scattered from an external source,

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<v Speaker 1>or they might emanate from the object itself. The color

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<v Speaker 1>of an object changes depending on the colors contained in

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<v Speaker 1>the light source. For example, red paint, when viewed under

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<v Speaker 1>blue light, looks black. Isaac Newton demonstrated with the prism

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<v Speaker 1>that the white light of the sun contains all colors

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<v Speaker 1>of the visible spectrum, so all colors are possible in sunlight.

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<v Speaker 1>In school, you may have learned that, for example, a

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<v Speaker 1>banana appears yellow because it reflects yellow light and absorbs

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<v Speaker 1>all other wavelengths. This isn't quite accurate, though, a banana

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<v Speaker 1>scatters as much orange and red as it does yellow,

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<v Speaker 1>and it scatters all of the colors of the visible

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<v Speaker 1>range to some degree or another. The real reason it

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<v Speaker 1>looks yellow relates to how our eyes sense light. Before

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<v Speaker 1>we get into that, however, let's look at what color

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<v Speaker 1>the sky actually is. Like bananas, atoms, molecules, and particles

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<v Speaker 1>in the atmosphere absorb and scatter light. If they didn't,

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<v Speaker 1>or if the Earth had no atmosphere, we would perceive

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<v Speaker 1>the Sun as a very bright star among others in

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<v Speaker 1>a sky of perpetual night. Not all wavelengths in the

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<v Speaker 1>visible light spectrum scatter equally. However, shorter and more energetic

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<v Speaker 1>wavelengths towards the violet end of the spectrum scatter better

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<v Speaker 1>than those towards the longer, less energetic red end. This

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<v Speaker 1>tendency is due in part to their higher energy, which

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<v Speaker 1>allows them to ping pong around more and in part

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<v Speaker 1>to the geometry of the particles that they interact with

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<v Speaker 1>in the atmosphere. In eighteen seventy one, Lord Rayleigh derived

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<v Speaker 1>a formula describing a subset of these interactions in which

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<v Speaker 1>atmospheric particles are much smaller than the wavelengths of the

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<v Speaker 1>radiation that are striking them. The Rayleigh scattering models showed

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<v Speaker 1>that in such systems, the intensity of scattered light is

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<v Speaker 1>inversely proportional to the fourth power of light's wavelength, which

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<v Speaker 1>is a really Matthew way of saying that shorter wavelengths

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<v Speaker 1>of light like blue and violet, scatter a lot more

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<v Speaker 1>than longer redder ones when the particles that they hit,

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<v Speaker 1>such as oxygen and nitrogen molecules, are relatively small. Under

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<v Speaker 1>these conditions, scattered light also tends to disperse equally in

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<v Speaker 1>all directions, which is why the sky appears so saturated

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<v Speaker 1>with color. If we were foolish enough to look directly

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<v Speaker 1>at the Sun, we would see all wavelengths because light

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<v Speaker 1>would be reaching our eyes directly. That's why the Sun

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<v Speaker 1>and the area around it look white. When we look

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<v Speaker 1>away from the Sun at the cloudless sky, we see

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<v Speaker 1>light mostly from shorter scattered wavelengths like violet, indigo, and blue.

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<v Speaker 1>So why doesn't the sky appear violet instead of light blue? Here?

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<v Speaker 1>The eyes have it. Your peepers perceive color using structures

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<v Speaker 1>called cones. Your retinas bristle with about five million cones,

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<v Speaker 1>each a made up of three types that specialize in

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<v Speaker 1>seeing different colors. Although each kind of cone is most

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<v Speaker 1>sensitive to certain peak wavelengths, the ranges of those cone

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<v Speaker 1>types overlap. As a result, different wavelengths of light and

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<v Speaker 1>combinations of different wavelengths can be detected as the same color.

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<v Speaker 1>Unlike our auditory senses, which can recognize individual instruments in

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<v Speaker 1>an orchestra, our eyes and brains interpret certain combinations of

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<v Speaker 1>wavelengths as a single, discreet color. Our visual sense interprets

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<v Speaker 1>the blue violet light of the sky as a mixture

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<v Speaker 1>of blue and white light, and that is why the

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<v Speaker 1>sky is light blue. Today's episode was written by Nicholas

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<v Speaker 1>Garbis and produced by Tyler Clang. For more on this

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<v Speaker 1>and lots of other curious topics, visit how stuffworks dot com.

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<v Speaker 1>Brain Stuff is production of I Heart Radio. For more

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