Question

Light incident normally on a plane mirror attached to a galvanometer coil retraces backwards as shown in Fig. 9.29. A current in the coil produces a deflection of 3.5o of the mirror. What is the displacement of the reflected spot of light on a screen placed 1.5 m away?

Answer 1

Angle of deflection, θ = 3.5°
Distance of the screen from the mirror, D = 1.5m
The reflected rays get deflected by an amount twice the angle of deflection i.e., 2θ = 7.0°
The displacement (d) of the reflected spot of light on the screen is given as :
$tan2θ = \frac{d }{ 1.5}$

$∴ d = 1.5 × tan7° = 0.184m = 18.4cm.$

Hence, the displacement of the reflected spot of light is 18.4 cm.