Two plane mirrors have an angle θ between their reflecting surfaces. A ray undergoes one reflection from each mirror in the same plane. What is the magnitude of the net rotation of its propagation direction?
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Question

Two plane mirrors have an angle θ between their reflecting surfaces. A ray undergoes one reflection from each mirror in the same plane. What is the magnitude of the net rotation of its propagation direction?

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G1 Physics · Accepted Answer

The composition of two reflections in lines intersecting at angle θ is equivalent to a rotation through 2θ. Hence the ray direction changes by 2θ, modulo 360°.