The angle of incidence and the angle of reflection are equal,
and they're both defined as the angle between the light ray
and the normal (perpendicular) to the mirror.
If the light ray comes in at an angle of 50° to the mirror, then the
angle of incidence is (90° - 50°) = 40°, the angle of reflection is
also 40°, and the light ray is redirected through a total angle of 80°.
