Question

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 35° of the mirror. What is the displacement of the reflected spot of light on a screen placed 1.5 m away?

Answer 1

When a plane mirror rotates through certain angle, the reflected ray turns through twice the angle of rotation. Therefore, angle between the incident ray AO and the reflected ray is,

θ = 35° x 2 = 70

from the right angled ΔOAB, we have

tan70 = \(\frac{AB}{AO}\)

or 0.1228 = \(\frac{d}{1.5}\)

or d = 0.1228 x 1.5

= 0.1842 m

= 18.42 cm