Visible light is made up of tiny particles called photons, these particles have different wavelengths depending on their colour: blue light comprises of particles with shorter wavelengths whereas red light is made of particles with longer wavelengths.

White light from the sun is made up of a continuous spectrum of colours which, conventionally, is divided into the colours of the rainbow (with progressively longer wavelengths: violet, indigo, blue, green, yellow, orange and red). It is the mixture of these colours that produces white.

However what happens to light when it travels through the atmosphere of the earth is that the shorter wavelengths of light become scattered. Our atmosphere is made from various gases and the atoms and molecules that these are formed from are suspended within it.

Photons travelling through the atmosphere physically collide with these atomic particles and a collision will deflect the photons and make them bounce in another direction. Shorter wavelengths are more likely to be deflected than longer ones, so that the photons which are scattered in all directions by these collisions are predominantly blue.