**Deviation of Light due to Rotation of a Mirror:** Let consider al ray of light AO incident on a plane mirror XY at O. It is reflected along OB. Let α be the glancing angle with XY (Figure). We know that the angle of deviation COB = 2α.

Suppose the mirror is rotated through an angle O to a position X’Y’.

The same incident ray AO is now reflected along OP. Here the glancing angle with X’Y’ is (α + O). Hence the new angle of deviation COP = 2 (α + θ). The reflected ray has thus been rotated through an angle BOP when the mirror is rotated through an angle θ.

**Angle BOP = Angle COP – Angle COB**

so, **Angle BOP = 2 (α + θ) – 2α = 2θ**

For the same incident ray, when the mirror is rotated through an angle, the reflected ray is rotated through twice the angle.