After heavy rainfall an coloured semicircles are seen in the sky is known as rainbow. The formation of rainbow is the effect of water droplets present in the atmosphere. But in hindu mythology it the bow of lord Indra.
The region behind the formation of rainbow is the entering of light ray in the water droplet and shows the various phenomenon of light.
When light ray entered in the waterdroplet their is refraction of light along with the scattering of white light into its various colours.
And when light ray leave after the reflection of light from the circular boundary of water froplet the light ray is coloured.
Their is overall phenomenon also called total internal reflection. The rainbow cause the formation of coloured semicircle in the sky sometimes it may give appearance of two or mote colours.