Question

why is rainbow usually seen after heavy rainfall

Answer 1

Each rain droplet has a function in the formation of the rainbow. Sunlight enters the rain droplet at a specific angle and therain droplet separates the white light into many different colors. This angle is a fixed measurement between your eye and the sun.

Answer 2

this is because when water droplets are present in atmosphere the sunlight passes through in such a way that refraction in the water droplet 2times internally leading to splitting of sunlight in its constituent colors in the form ofVIBGYOR.