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

The correct option is B

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

For min deviation

i = e

$r_1=r_2=\frac{A}{2}$

$\therefore r_1=r_2={30}^{\circ }$

For minimum deviation i should lie between ${40}^{\circ }$ to ${50}^{\circ }$.