On a clear day the sky is a bright blue, yet sunlight itself is white. So where does the blue come from? The answer lies in how sunlight interacts with the air.
Sunlight is a mix of colours
White sunlight contains all the colours of the rainbow. Each colour has a different wavelength: red light has long waves and blue and violet light have short waves.
Air molecules scatter short waves
Our atmosphere is mostly nitrogen and oxygen molecules, which are far smaller than the wavelength of light. When sunlight hits them, the light is scattered in all directions. This is called Rayleigh scattering, and it affects short wavelengths much more strongly than long ones. Blue light is scattered roughly five times more than red light.
So as sunlight passes through the air, blue light is bounced around the whole sky. Wherever you look, some of that scattered blue light reaches your eyes, and the sky appears blue.
Why not violet?
Violet light is scattered even more than blue, so why is the sky not violet? There are three reasons: the Sun gives off less violet than blue, some violet is absorbed high in the atmosphere, and our eyes are more sensitive to blue.
Why are sunsets red and orange?
At sunrise and sunset, sunlight travels through much more air to reach you. By the time it arrives, most of the blue has been scattered away, leaving the reds, oranges and yellows. Dust, smoke and pollution can make sunsets even more colourful.
What colour is the sky on other planets?
- Moon: black, because there is no atmosphere to scatter light.
- Mars: a butterscotch colour by day because of fine dust, while Martian sunsets look bluish.
