Explain Deviation of Light due to Rotation of a Mirror - QS Study
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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.

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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.