A small angled prism of apex angle A = 4° and refractive index μ = 1.5 is placed in front of a vertical plane mirror as shown in figure. If the mirror is rotated through an angle $\theta$, then the light ray becomes horizontal either after the mirror or after second time refraction through the prism. The value of $\theta$ is

Deviation, $\delta = (\mu -1)A = (1.5-1) 4° = 2°$

(a)

To rotate ray MP by 2° (to make it parallel to MN) we will have to rotate the mirror only by 1°
(b) Without prism, ray of light was falling normal to plane mirror. So, ray of light was retracing its path prism has deviated it 2°. So if we rotate the mirror by 2°, ray of light further falls normal to plan mirror and retraces its path.