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.