Look at a clear sky. You are seeing sunlight bounce off gas. The air is about 78 percent nitrogen and about 21 percent oxygen. The rest is argon, carbon dioxide and other gases. These gases do the work of coloring the sky.

The light they work on is sunlight, which looks white but is really made up of all the colors of the rainbow. A glass prism can split white light into those colors. Isaac Newton used a prism to show that white sunlight is a mixture of all the colors. A rainbow shows the same thing: that sunlight is made of many colors.

Visible light is the part of the electromagnetic spectrum that human eyes can see, 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. Red light has the longest waves, and blue and violet light have much shorter waves. Blue waves are shorter than red waves.

This is called Rayleigh scattering. 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. Nitrogen and oxygen molecules are the main gases that scatter the light. They are too small to see.

Violet has the shortest visible waves, so you might expect a violet sky. Violet light is scattered even more than blue, yet the sky is blue. Why? Sunlight has less violet in it, and our eyes see violet less well. Our eyes have three kinds of color receptors, called cones, in the retina.

At sunset the same process is at work. The low Sun's light crosses more air, scattering out most of the blue and leaving reds and oranges. 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.

Clouds themselves look white. You may have heard that clouds are white because water is white. That is not the reason. Cloud droplets and ice crystals are much larger than the wavelength of visible light, so they scatter all colors about equally. They are not white because water is white.

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

So the sky's colors all come from sunlight meeting air. Blue skies and red sunsets come from the same scattering process. One rule goes with every sky: never look straight at the Sun. Even dark sunglasses do not make it safe for your eyes.

Words to know

• nitrogen: main air gas
• scattering: bouncing about
• molecules: tiny gas bits
• nanometers: tiny units