Question

A ray of light PQ is incident on an equilateral prism ABC of refarctive index √3 . It passes through the prism ABC as shown in Fig. CDE is a second prism exactly identical to prism ABC. The emergent ray, from prism ABC, is required undergo minimum deviation in prism CDE. For this the prism CDE should be rotated through

(1) 30° , clockwise

(2) 60° , clockwise

(3) 60° , anticlockwise

(4) 30° , anticlockwise

Answer 1

(3) 60° , anticlockwise

The path, QR, of ray inside prism ABC, is parallel to base AC of prism. The incident ray PQ is therefore, undergoing minimum deviation in this prism. Let i₁ and r₁ denote angle of incidence and refraction, at the face on AB. For minimum deviation,

The ray QR emerges out of prism from face BC; at angle of emergence = 60° . It is, therefore, incident normally, on face CD, of prism CDE. If this ray is to undergo minimum deviation inside CDE, angle of incidence of RS, on CD, should be again 60° . This will be so if prism CDE is rotated, through 60° , in the anticlockwise direction.