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

Angle of deflection, 0 = 3.5°

Distance of the screen from the mirror, D = 1.5 m

The reflected rays get deflected by an amount twice the angle of deflection i.e., 20 = 7.0°

The displacement (d) of the reflected spot of light on the screen is given as:

tan 20 = d/1.5 => d = 1.5 x tan 7° = 0.184 m = 18.4 cm

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