When white light from sun falls on rain drops, sometimes a band of different colours in form of a circular arc is seen in the sky opposite the sun. This is called the rainbow.

The reason of origin of rainbow is that the small drops of water behave like a prism for the white sun light due to which refraction, dispersion and total internal reflection of white light occurs from the water drops.

The rainbow is not seen after every rain, but is seen only when the light rays of particular colour suffer minimum deviation after one or two total internal reflections inside the small water drops.