Light incident normally on a plane mirror attached to a galvanometercoil retraces backwards as shown in Fig. 9.36. A current in the coil produces a deflection of 3.5o of the mirror. What is the displacementof the reflected spot of light on a screen placed 1.5 m away?

Given,

Deflection of the mirror, θ=3.5°

Since, a deflection of θ of the plane mirror deflects the reflected ray by 2θ, the deflection of the reflected ray is,

θ d =2×3.5° =7°

Let S be the displacement of reflected ray placed 1.5 maway.

Thus,

S=1.5×7( π 180 ) =0.184 m =18.4 cm