Question

When a plane mirror is rotated through an angle theta the reflected ray turns through?

Answer 1

A ray of light is reflected by an angle (angle of incidence or angle of reflection; since both are equal) about the normal after reflection. Now, when the mirror itself is rotated by theta, the normal also rotates by theta. So the ray of light is reflected at an angle theta about the Normal.

Answer 2

Then normal Ray also rotates to theta

So the Ray of light reflected at an angle theta about to normal