PHYSICS · Single correct · AP · Physics 2: Algebra-based · Geometric Optics · Geometric Optics
Problem
Two plane mirrors meet at right angles. A light ray strikes mirror M1, reflects to mirror M2, and then travels directly back toward its source. Which of the following best explains why the ray retraces its path regardless of the angle at which it first meets mirror M1?
(a) Because the net deviation after two reflections equals the 90° angle between the mirrors, the ray turns exactly one right angle and returns to its source.
(b) The first reflection forces the ray to strike the second mirror at normal incidence, so the second mirror sends it straight back along its previous path.
(c) Successive reflections from perpendicular mirrors always reverse both components of the ray’s velocity, sending it back along the incoming line.
(d) Each reflection reverses the entire velocity vector, so two reflections cancel out and the ray continues in its original direction.