Deviation of Reflected Ray by a Plane Mirror

Here AO is a incident ray that incidents on reflecting surface DE and passes through OB. Let OH be the original path of incident ray. Let θ be the angle of deviation. In first case,

2θ = δ [since angle of deviation is twice the glancing angle.]


Again, the mirror is rotated through small angle. Let the new angle of reflection be β . Now,

∠IOH = 2(θ + β)

∴ ∠IOH = 2θ + 2β

Now, deviation produced = 2θ + 2β – 2β

∴ Deviation produced = 2θ

Hence, we conclude, the law of rotation of light states that when a mirror is rotated to θº, then the reflected ray is rotated through 2θº  keeping angle of incidence constant.

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