Why Is the Sky Blue — and Why Does It Turn Orange at Sunset?

Step outside on a clear afternoon and look up. The sky is blue. Wait a few hours and look again — now it is orange, pink and gold. Nothing about the Sun has changed. So what has?

Sunlight is a mixture

Sunlight looks white, but it is really all the colours of the rainbow travelling together. You can see them separate when light passes through a prism, or through raindrops after a shower — that is exactly what a rainbow is.

Each colour travels as a wave, and the waves are not all the same size. Red light has long, lazy waves. Blue and violet light have short, quick ones.

The air gets in the way

Our atmosphere is full of tiny molecules of gas — far too small to see. When sunlight hits them, the light bounces off in all directions. Scientists call this scattering. Here is the important part: the small, quick blue waves get bounced around far more than the long red ones.

So blue light gets thrown across the whole sky, arriving at your eyes from every direction at once. That is why the sky above you glows blue even though the Sun is only in one place.

Sunset: the light takes the long way round

At sunset the Sun sits low, so its light has to travel through much more air to reach you — a long slanting path instead of a short straight one. Along that journey almost all the blue is scattered away before it arrives. What is left to reach your eyes is the light that scatters least: orange and red.

The sky is not painted. It is sorted — by size of wave.