Question

what is the reason for rainbow formation?

Answer 1

the rainbow form due to scattering of light

Answer 2

after the rain , all the raindrops in the atmosphere act as many small prism.
as white light from sun enters and leaves these raindrops, various coloured rays present in white light are refracted by different amounts due to which an arch of seven colours called rainbow is formed in the sky.

hopes it hlps u.