Sunlight looks white, but it is really made up of all the colors of the rainbow. A glass prism can split white light into those colors, and Isaac Newton used a prism to split sunlight into a spectrum. That showed that white light is a mixture of colors, and a rainbow shows the same thing, that sunlight is made of many colors. Each color in a rainbow corresponds to a different wavelength of electromagnetic radiation, and that fact is the key to the color of the sky.

Human eyes can see only part of the electromagnetic spectrum, which we call visible light, with wavelengths of roughly 380 to 700 or 750 nanometers. Violet has the shortest visible wavelength, near 380 nanometers, and red has the longest, between about 700 and 750 nanometers. Blue light waves are shorter than red light waves, so the two colors behave differently when they meet the air.

The air is not empty. It holds tiny bits that are too small to see, and they bounce sunlight around the sky. Those bits are molecules. About 78 percent of the air is nitrogen and about 21 percent is oxygen, and the rest is argon, carbon dioxide and other gases. So the molecules that scatter sunlight are mostly nitrogen and oxygen.

The scattering of light by particles much smaller than the wavelength of the light, such as single air molecules, is called Rayleigh scattering. It is named after the 19th-century British physicist Lord Rayleigh. Light that has been scattered this way travels in all directions. That is why a patch of sky far from the Sun still shines with blue light.

The amount of light scattered is inversely proportional to the fourth power of its wavelength, so shorter waves scatter far more. Because of this fourth-power rule, blue light, at about 400 nanometers, is scattered about ten times more than red light, at about 700 nanometers. Blue light gets bounced around the sky more than the other colors, so the sky looks blue.

There is a puzzle in this. Violet is scattered even more strongly than blue, and if Rayleigh scattering were the only thing going on, the sky would look violet instead of blue. Two other facts settle it. Sunlight contains less violet than blue, and human vision is less sensitive to violet wavelengths than to blue ones. So, taken together, what we see overhead is a blue sky, and it took all of these facts together to explain it.

Look toward the horizon and the blue fades. Near the horizon the sky is lighter and whiter than overhead. Closer to the horizon, light travels through more air and is scattered and re-scattered, which mixes in white light and makes the blue paler.

The same idea explains sunsets. When the Sun is low in the sky, its light has to go through more air to reach your eyes. At sunrise and sunset the low Sun's light passes through a thicker layer of atmosphere, so more of the blue is scattered out of the beam. The reds and oranges of sunset are the longer wavelengths that remain after the shorter ones have been removed. Blue skies and red sunsets come from the same scattering process. Clouds near the horizon at sunset can glow red or orange because they reflect the reddened sunlight. Never look straight at the Sun. Even dark sunglasses do not make it safe for your eyes.

Clouds do not turn blue, and the reason is size. Cloud droplets and ice crystals, about 10 micrometers or larger, are much larger than the wavelengths of visible light, so they scatter all wavelengths about equally. Clouds look white because the tiny drops in them bounce all the colors of light about equally.

Other worlds show what the air does. The Moon has only a very thin exosphere and no air to scatter sunlight, so its sky looks black even in the daytime. Mars has a thin atmosphere too, and its daytime sky is darker than Earth's because that air is so thin. It is mostly carbon dioxide, nitrogen and argon. Yet a NASA rover took a color picture of a sunset on Mars, and the sky around the setting Sun looked blue.

One more clue hides in the sky. The light of the sky is polarized, which a polarizing filter can show. The next time you look up at the sky on a clear day, remember that you are seeing sunlight that air molecules have sent in every direction, mostly blue.

Words to know

• scattering: bouncing everywhere
• wavelength: length of wave
• molecules: tiny particles
• polarized: seen with filter