The sky is blue, the Sun looks white, and a sunset can burn orange and red. These colors seem to have nothing to do with each other. They come from one thing: sunlight meeting the air. Sunlight looks white, yet the sky has its own colors, so something must happen to the light on its way down. Let's follow the light and find out what happens to each color. So the best way to understand the sky is to follow one beam of sunlight from the Sun to your eyes. Here is how that trip works, step by step, and what happens to each color along the way.

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 show that white sunlight is a mixture of all the colors. A rainbow shows the same thing in the sky. So white light is not a plain thing. It is a mix, and the sky will pull that mix apart.

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. Blue light waves are shorter than red light waves, and that gap is what matters for the sky. Violet sits at the short end. Red sits at the long end. Our eyes can see only the colors in between.

Now add the air. It has tiny bits in it, called molecules, and most of them are nitrogen and oxygen. They are too small to see, but they bounce sunlight around the sky. Nitrogen and oxygen molecules are the main gases in the air that scatter the light.

Scientists call this 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. That is why the sky looks blue.

Violet has even shorter waves, and violet light is scattered even more than blue. So why is the sky not violet? Sunlight has less violet in it, and our eyes see violet less well. Our eyes also have three kinds of color receptors, called cones, in the retina. These receptors are the parts of the eye that sense color.

Sunsets use the same scattering, and at sunset the sky turns orange and red. When the Sun is low in the sky, its light has to go through more air to reach your eyes. Sunsets are red and orange because the low Sun's light crosses more air, scattering out most of the blue and leaving reds and oranges. Never look straight at the Sun. Even dark sunglasses do not make it safe for your eyes.

This means blue skies and red sunsets come from the same scattering process.

Clouds follow a different rule. You may have heard that clouds are white because water is white. In fact, cloud droplets and ice crystals are much larger than the wavelength of visible light, so they scatter all colors about equally and clouds look white.

The Moon shows what happens with no air. It has only a very thin exosphere and no air to scatter sunlight, so its sky looks black even in the daytime. With no air, nothing bounces the light around, so the Moon has no blue sky. On Earth, the air scatters blue light, and blue skies and red sunsets come from the same scattering process.

Words to know

• wavelength: a wave's length
• nanometers: tiny units
• scattering: bouncing about
• retina: in the eye
• cones: color receptors