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. 

Answer 2

as we know that angle of incidence is equal to the angle of reflection,therefore, angle of reflection will also be perpendicular to the mirror. Or, we can say Li=Lr=90°.

Hope it helps you!