Why is the sunset red?

A lesson in colour

To begin we need to understand how we see colour. Light travels from the sun and through the atmosphere in invisible waves. Light that looks white to us, is actually made up of many different colours of differing wavelengths. For example blue has a short wavelength, while red is created by longer wavelengths.

Why is the sunset red or orange?

At sunrise and sunset the sun is very low in the sky, which means that the sunlight we see has travelled through a much thicker amount of atmosphere. Because blue light is scattered more strongly by the atmosphere, it tends to be scattered several times and deflected away in other directions before it gets to us. This means that there is relatively more yellow and red light left for us to see.

The diagram on this page shows a simplified illustration of these effects. A person standing at position A would see a blue daytime sky, as there is plenty of blue light being scattered in all directions.

At position B, it is evening and a person standing here would see a familiar orange sunset because the blue light has mostly been scattered away in other directions, leaving the reds and yellows.