Light incident normally on a plane mirror attached to a galvanometer coil retraces backwards as shown in Fig. 9.36.

Angle of deflection, θ = 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., 2θ= 7.0°

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

$\tan 2 \theta=\frac{d}{1.5}$

$\therefore d=1.5 \times \tan 7^{\circ}=0.184 \mathrm{~m}=18.4 \mathrm{~cm}$

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