Speaker 1: Welcome to Brainstuff, a production of iHeart Radio. Hey, brain Stuff.
Speaker 1: Lauren bogelbam here. It seems like a simple question, but
Speaker 1: it took many centuries and a lot of smart people,
Speaker 1: including Aristotle, Isaac Newton, Thomas Young, James Clerk, Maxwell, and
Speaker 1: Herman von Helmholtz to puzzle out the answer to why
Speaker 1: is the sky blue? That's because the solution encompasses so
Speaker 1: many components, the colors in sunlight, the angle at which
Speaker 1: solar illumination travels through the atmosphere, the size of airborne
Speaker 1: particles and atmospheric molecules, and the way our eyes perceive color.
Speaker 1: Let's take the sky out of the equation for a
Speaker 1: moment and begin by looking at color from a physics standpoint.
Speaker 1: Color refers to the wavelengths of visible light leaving an
Speaker 1: object and striking a sensor such as a human eye.
Speaker 1: These wavelengths might be reflected or scattered from an external source,
Speaker 1: or they might emanate from the object itself. The color
Speaker 1: of an object changes depending on the colors contained in
Speaker 1: the light source. For example, red paint, when viewed under
Speaker 1: blue light, looks black. Isaac Newton demonstrated with the prism
Speaker 1: that the white light of the sun contains all colors
Speaker 1: of the visible spectrum, so all colors are possible in sunlight.
Speaker 1: In school, you may have learned that, for example, a
Speaker 1: banana appears yellow because it reflects yellow light and absorbs
Speaker 1: all other wavelengths. This isn't quite accurate, though, a banana
Speaker 1: scatters as much orange and red as it does yellow,
Speaker 1: and it scatters all of the colors of the visible
Speaker 1: range to some degree or another. The real reason it
Speaker 1: looks yellow relates to how our eyes sense light. Before
Speaker 1: we get into that, however, let's look at what color
Speaker 1: the sky actually is. Like bananas, atoms, molecules, and particles
Speaker 1: in the atmosphere absorb and scatter light. If they didn't,
Speaker 1: or if the Earth had no atmosphere, we would perceive
Speaker 1: the Sun as a very bright star among others in
Speaker 1: a sky of perpetual night. Not all wavelengths in the
Speaker 1: visible light spectrum scatter equally. However, shorter and more energetic
Speaker 1: wavelengths towards the violet end of the spectrum scatter better
Speaker 1: than those towards the longer, less energetic red end. This
Speaker 1: tendency is due in part to their higher energy, which
Speaker 1: allows them to ping pong around more and in part
Speaker 1: to the geometry of the particles that they interact with
Speaker 1: in the atmosphere. In eighteen seventy one, Lord Rayleigh derived
Speaker 1: a formula describing a subset of these interactions in which
Speaker 1: atmospheric particles are much smaller than the wavelengths of the
Speaker 1: radiation that are striking them. The Rayleigh scattering models showed
Speaker 1: that in such systems, the intensity of scattered light is
Speaker 1: inversely proportional to the fourth power of light's wavelength, which
Speaker 1: is a really Matthew way of saying that shorter wavelengths
Speaker 1: of light like blue and violet, scatter a lot more
Speaker 1: than longer redder ones when the particles that they hit,
Speaker 1: such as oxygen and nitrogen molecules, are relatively small. Under
Speaker 1: these conditions, scattered light also tends to disperse equally in
Speaker 1: all directions, which is why the sky appears so saturated
Speaker 1: with color. If we were foolish enough to look directly
Speaker 1: at the Sun, we would see all wavelengths because light
Speaker 1: would be reaching our eyes directly. That's why the Sun
Speaker 1: and the area around it look white. When we look
Speaker 1: away from the Sun at the cloudless sky, we see
Speaker 1: light mostly from shorter scattered wavelengths like violet, indigo, and blue.
Speaker 1: So why doesn't the sky appear violet instead of light blue? Here?
Speaker 1: The eyes have it. Your peepers perceive color using structures
Speaker 1: called cones. Your retinas bristle with about five million cones,
Speaker 1: each a made up of three types that specialize in
Speaker 1: seeing different colors. Although each kind of cone is most
Speaker 1: sensitive to certain peak wavelengths, the ranges of those cone
Speaker 1: types overlap. As a result, different wavelengths of light and
Speaker 1: combinations of different wavelengths can be detected as the same color.
Speaker 1: Unlike our auditory senses, which can recognize individual instruments in
Speaker 1: an orchestra, our eyes and brains interpret certain combinations of
Speaker 1: wavelengths as a single, discreet color. Our visual sense interprets
Speaker 1: the blue violet light of the sky as a mixture
Speaker 1: of blue and white light, and that is why the
Speaker 1: sky is light blue. Today's episode was written by Nicholas
Speaker 1: Garbis and produced by Tyler Clang. For more on this
Speaker 1: and lots of other curious topics, visit how stuffworks dot com.
Speaker 1: Brain Stuff is production of I Heart Radio. For more
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