Leonardo da Vinci tried to explain the sky's color as early as about 1500. Isaac Newton used a prism to split sunlight into a spectrum, showing that white light is a mixture of colors, and Thomas Young was the first to measure the wavelengths of different colors of light, in 1802. 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, an effect that can be shown by shining a beam of white light through a tank of water with a little milk or soap mixed in. Lord Rayleigh worked out much of the scattering explanation in the 1870s, and the scattering is named after him.

Rayleigh scattering 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, and because of the fourth-power rule, blue light, at about 400 nanometers, is scattered about ten times more than red light, at about 700 nanometers. 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. Sunlight contains less violet than blue, and, as a second reason, human vision is less sensitive to violet wavelengths than to blue ones. A blue rather than violet sky needs several effects together.

Light that has been scattered this way travels in all directions, so a patch of sky far from the Sun still shines with blue light and gives diffuse sky radiation. Scattered blue light is polarized. The light of the sky is polarized, which a polarizing filter can show. Sky light is most strongly polarized at points 90 degrees from the Sun. Many insects, including bees, can sense the polarization of skylight. The sky is blue because of Rayleigh scattering, where gas molecules in the air send short blue waves of sunlight in all directions.

The ozone layer was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson. Fabry and Buisson found that sunlight reaching the ground had no radiation below about 310 nanometers, and the spectrum of the missing radiation matched only one known chemical, ozone. Ozone is a gas made of three oxygen atoms (O3). Ozone in the stratosphere is made when ultraviolet light strikes ordinary oxygen, and the ozone layer in the stratosphere absorbs most of the Sun's ultraviolet radiation. The ozone layer lies somewhere between about 9 and 30 miles above the ground, in the stratosphere.

At sunset the sky turns orange and red, because 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. If the air is polluted with small particles, natural or otherwise, the sunset will be more red. During the blue hour, the deep blue of the sky overhead comes from ozone in the upper atmosphere absorbing red light. The blue hour lasts about 20 to 30 minutes at mid-latitudes, can last hours in the polar night, and is shorter near the equator where the Sun sets steeply. Never look straight at the Sun. Even dark sunglasses do not make it safe for your eyes.

Mie scattering describes light scattered by particles whose size is comparable to the wavelength of the light, such as the droplets in a cloud. Mie scattering 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. The Rayleigh model stops working when particles grow larger than about 10 percent of the wavelength of the light. Clouds look white or grey because the droplets scatter in the Mie way, roughly equally at all wavelengths, and the tiny drops in a cloud bounce all the colors of light about equally.

The ocean is blue because water absorbs the red part of the light spectrum. Water in the ocean can also look green, red or other hues when light bounces off sediments and particles floating in it. On Uranus, methane gas absorbs the red part of sunlight, giving the planet a blue-green color. The sky is blue for a different reason: Rayleigh scattering, where gas molecules in the air send short blue waves of sunlight in all directions. Blue light is scattered much more than red light because blue has shorter waves.

Words to know

• polarized: waves lined up
• ozone: three-oxygen gas
• absorbing: taking in
• stratosphere: upper air layer
• sediments: settled bits