Question

A horizontal ray of light passes through a prism of index 1.50 and apex angle 4ŗ and then strikes a vertical mirror, as shown in the figure (1). Through what angle must the mirror be rotated if after reflection the ray is to be horizontal?

• A. 1°
• B. 2°
• C. 2.5°
• D. 1.5°

Answer 1

Answer: (A)

1°

Answer 2

As the apex angle is very small (a = 4⁰), the angle of deviation & can be obtained approximately:

d = (n – 1)a = (1.5 – 1) × 4⁰ = 2⁰.

Fig. (2)

From Fig. (2) we see that if the reflected ray is to be horizontal, the mirror must be rotated clockwise through an angle g given by

g =$\frac { \delta } { 2 }$= 1⁰