On a clear day, the sky above us is blue, but the reason is more surprising than it first seems. Sunlight may look plain and white, yet it is actually a mixture of many colours, just like the colours in a rainbow. Red, yellow, green and blue light are all travelling together inside a single beam of sunlight.
When this light enters the atmosphere, it meets countless tiny molecules of gas. These molecules scatter the light, pushing it off in different directions. Crucially, they scatter short, blue waves of light much more strongly than longer, red ones. As a result, blue light is spread right across the sky, and wherever you look, some of it reaches your eyes. That is why the whole sky appears blue rather than the colour of the Sun itself.
The same process explains why sunsets are red. When the Sun is low, its light must pass through a much thicker layer of air. By the time it reaches you, most of the blue has been scattered away, leaving the warm reds and oranges we associate with the end of the day.
