Question

what will be angle of reflection if a Ray of light strikes a plane mirror at an angle of 90°

Answer 1

By the law

angle of incidence = angle of reflection

angle of reflection =90°

90° in a plane mirror will be 0° also so both are correct answers

Answer 2

your ans

as we know that laws of reflection says that

angle of incidence = angle of reflection

so when a ray will strike normally (90°) on a plane mirror its angle of incidence will be 0 so its angle of reflection will also be 0