Question

Rainbow is a phenomenon due to

Answer 1

Rainbow is a phenomenon due to splitting of a ray of white light into it's constituent coloured shortformed as VIBGYOR

Answer 2

Rainbow is a phenomenon or literally a spectrum of light that forms as a result of dispersion, reflection and refraction of light from the water droplets in the sky. Rainbows can be found always in the opposite side of the sun in the sky with multicolored arc.  

This is how rainbows are formed:

When a ray of light enters through a droplet of water in the sky the light will be refracted initially and reflected once it get inside the droplet and tends to refract again when leaving the droplet.