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 $40^{\circ }<i<50^{\circ }$.