Isaac Newton used a prism to split sunlight into a spectrum, showing that white light is a mixture of colors. A glass prism can split white light into the colors of the rainbow, and a rainbow shows that sunlight is made of many colors. Each color in a rainbow corresponds to a different wavelength of electromagnetic radiation. Thomas Young was the first to measure the wavelengths of different colors of light, in 1802.

Many people think sunlight is a perfectly white color, but the Sun gives out more yellow light than any other color. A rainbow shows that there is still a significant amount of violet and indigo light in sunlight beyond the blue, so a lack of violet in sunlight is only part of the story.

The physicist John Tyndall took some of the first steps toward explaining why the sky is blue, in the 1800s. In 1869 Tyndall found that bright light scattered off tiny particles looks faintly blue-tinted. The scattering of light by tiny particles can be shown by shining a beam of white light through a tank of water with a little milk or soap mixed in.

The air has tiny bits in it that are too small to see, and they bounce sunlight around the sky. The bits of air that bounce the light are called molecules, and the air molecules that scatter sunlight are mostly nitrogen (N2) and oxygen (O2). About 78 percent of the air is nitrogen and about 21 percent is oxygen; the rest is argon, carbon dioxide and other gases. Light that has been scattered by these molecules travels in all directions, so a patch of sky far from the Sun still shines with blue light and gives diffuse sky radiation.

Rayleigh scattering is named after the 19th-century British physicist Lord Rayleigh. It is the scattering of light by particles much smaller than the wavelength of the light, such as individual air molecules. Under Rayleigh scattering the amount of light scattered is inversely proportional to the fourth power of its wavelength, so shorter waves scatter far more. Blue light, at about 400 nanometers, is scattered about ten times more than red light, at about 700 nanometers, and blue light is scattered much more than red light because blue has shorter waves.

If Rayleigh scattering were the only thing going on, the sky would look violet instead of blue. Violet is scattered even more strongly than blue, yet the sky does not look violet because sunlight contains less violet than blue. Human vision is less sensitive to violet wavelengths than to blue ones. Our eyes have three kinds of color receptors, called cones, and our brains mix the spectrum of skylight into the sky blue we see. A blue rather than violet sky needs several effects together: the scattering, how little violet sunlight holds, and how our eyes respond.

Look toward the horizon and the blue changes. Near the horizon the sky fades to a lighter blue or white. Closer to the horizon, light travels through more air and is scattered and re-scattered, mixing in white light and making the blue paler. Looking toward the horizon the sky is lighter and whiter than overhead, because the longer path scatters and re-scatters the light. 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 Rayleigh scattering has removed the shorter ones from the beam. 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 follow a different kind of scattering. Mie scattering describes light scattered by particles whose size is comparable to the wavelength of the light, such as the droplets in a cloud. It is named after the German physicist Gustav Mie, who developed the solution for the scattering of a light wave by a sphere. The dominant scattering processes in the atmosphere are Rayleigh scattering and Mie scattering. Clouds look white or grey because the droplets scatter in the Mie way, roughly equally at all wavelengths.

The Rayleigh model stops working when particles grow larger than about 10 percent of the wavelength of the light. The sky is blue because of Rayleigh scattering, where gas molecules in the air send short blue waves of sunlight in all directions. The ocean is blue for a different reason, because water absorbs the red part of the light spectrum. One blue comes from scattering, and the other comes from absorption.

Words to know

• spectrum: band of colors
• wavelength: length of wave
• molecules: tiny particles
• absorbs: takes in
• scattering: bouncing everywhere