Question

What happens when a ray of light is fall on a plain mirror???

Answer 1

Visible light photons don't react with each other when they collide. They just pass through each other. I think that you want an answer which is more explanatory, like when a continuous beam of light falls normally and gets reflected,do the incoming and reflected light interact in any way.

Answer 2

when the ray of light fall on the plane mirror. It will be reflected and go back.