When you see a rainbow

A rainbow is a meteorological phenomenon formed when light, generally from the Sun, passes through water droplets hanging in the atmosphere. The atmosphere is a layer of gas that surrounds the Earth. It enables us to breathe and protects us from harmful radiation emitted by the Sun.
To be able to see a rainbow, you normally need to have your back to the Sun, because rainbows will usually appear in the section of sky directly opposite the Sun.
A rainbow is the result of three scientific phenomena: reflection, refraction and dispersion of light.
Refraction is the change in the speed of a wavelength while it passes through a material. This is most commonly seen with light and transparent objects, such as glass or water. To create a rainbow, white light from the Sun enters the droplet of water and is refracted.

It causes dispersion in the same way in which white light generates a spectrum of light when it goes through a prism.
At the back of the droplet of water, this spectrum is reflected and refracted once again when leaving the droplet.

Every colour in a ray of light has a different wavelength. Because of this, when it's refracted, each one will change speed slightly and bend to a different degree, separating the colours and making them visible to our eyes