A ray makes 30° with a plane mirror surface. Keeping the mirror fixed, the incident ray is rotated so that the angle between the incident and reflected rays becomes 100°. By how many degrees has the angle of incidence changed?
Physics · Geometrical optics · law of reflection and plane mirror
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Question

A ray makes 30° with a plane mirror surface. Keeping the mirror fixed, the incident ray is rotated so that the angle between the incident and reflected rays becomes 100°. By how many degrees has the angle of incidence changed?

Physics · Geometrical optics · law of reflection and plane mirror

G1 Physics · Accepted Answer

Initially the angle of incidence is 60°. The smaller angle between incident and reflected rays is 2i, so for 100° separation the new incidence angle is 50°. The change in incidence angle is |60°−50°|=10°. If ray-direction rotation is measured relative to the original incident direction toward the normal, it is 10°.