Question

A ray of light is incident normally (perpendicularly) on a plane mirror. Where will this ray of light go after reflection from the mirror?

Answer 1

For a plane mirror: If the incident ray falls along the normal, the angle of incidence is 0 degrees, not 90 degrees. The angle is measured with the normal to the mirror and not with the plane of the mirror. Hence the angle of reflection is equal to 0 degrees. So it retraces the path.