Light reflection in plane mirrors MCQ - Practice Questions with Answers

The angle of incidence is measured from the normal to the mirror, not from the mirror surface. If the incident ray makes 15° with the normal, then the same geometry leaves 90° - 15° = 75° with the plane mirror surface. Hence angle X, as indicated in the figure, is 75°. The values 45° and 65° do not follow from the complementary angle relation here. The value 85° would correspond to only a 5° angle with the normal, which is not the given incidence angle.

The angle of incidence is measured between the incident ray and the normal to the reflecting surface. If the ray falls normally on a plane mirror, it travels along the normal itself, so the angle of incidence is 0°. By the law of reflection, the angle of reflection equals the angle of incidence; therefore the reflected ray also makes 0° with the normal and retraces the same path. Choices containing 90° confuse the normal with the mirror surface, so they do not match the definition of the angle.