Question

The angle of a prism is $60^{\circ}$. When light is incident at an angle of $60^{\circ}$ on the prism, the angle of emergence is $40^{\circ}$. The angle of incidence $i$ for which the light ray will deviate the least is such that

• A. $i < 40^{\circ}$
• B. $40^{\circ} < i < 50^{\circ}$
• C. $50^{\circ} < i < 60^{\circ}$
• D. $i > 60^{\circ}$

Answer 1

Answer: (B)

Graph of deviations $\delta$ versus angle of incidence $i$ is as shown below (for an equilateral glass, $\mu = \frac{3}{2}$ prism).

Clearly, angle of incidence for least deviation lies between $40^{\circ} < i < 50^{\circ}$.