Look up on a sunny day and the sky is blue. But sunlight itself looks white. So where does the blue come from? The answer is a story about light, air and our eyes. It starts with the fact that sunlight is made of many colors, and it ends with a sky that looks blue.

How do we know this? Isaac Newton used a prism to show that white sunlight is a mixture of all the colors. A glass prism can split white light into the colors of the rainbow. Sunlight looks white, and it is really made up of every color of the rainbow.

Each color is a different kind of light wave. Red light has the longest waves. Blue and violet light have much shorter waves. Blue waves are shorter than red waves, and that fact turns out to matter a lot.

The air around us has tiny bits in it that are too small to see. They are called molecules, and most of them are nitrogen and oxygen. Nitrogen and oxygen molecules are the main gases in the air that scatter the light. That means they send it off in many directions. The air is all around you, yet you cannot see the bits in it. Scatter means to bounce sunlight around the sky.

Scientists call what happens next 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.

So blue light gets bounced around the whole sky more than the other colors, and the sky looks blue. Look near the horizon and you will see the sky fade to a lighter blue or white.

You might wonder about violet. Violet light is scattered even more than blue. But sunlight has less violet in it, and our eyes see violet less well. So we see a blue sky, not a violet one.

Clouds are different. You may have heard that clouds are white because water is white. In fact, clouds look white because the tiny drops in them bounce all the colors of light about equally. Blue is not picked out, so the cloud does not look blue.

Here is a test of the idea. The Moon has only a very thin exosphere and no air to scatter sunlight, so its sky looks black even in the daytime. With nothing to scatter the light, there is no blue sky at all.

At sunset the Sun is low in the sky, so its light has to go through more air to reach your eyes. The sky turns orange and red. Never look straight at the Sun, though. Even dark sunglasses do not make it safe for your eyes.

Words to know

• scattering: bouncing about
• molecules: tiny gas bits
• prism: light splitter
• violet: purple light
• horizon: edge of sky

The short version

The sky is blue because of Rayleigh scattering. Gas molecules in the air, mainly nitrogen and oxygen, send short blue waves of sunlight in all directions. Blue light is scattered much more than red light because blue has shorter waves. At sunset the light crosses more air and the sky turns orange and red.

Quick facts

• Sunlight looks white but is made up of all the colors of the rainbow.
• Blue light is scattered much more than red light because blue has shorter waves.
• Violet is scattered even more, but sunlight has less violet and our eyes see it less well.
• Nitrogen and oxygen molecules are the main gases that scatter the light.
• The Moon has no air to scatter sunlight, so its daytime sky looks black.