Question

A beam of monochromatic light is incident on one face of an equilateral prism. The angle of incidence is $ 55{}^\circ $ . If the angle of emergence is $ 46{}^\circ $ , then the angle of minimum deviation $ {{\delta }_{m}} $ will be

• A. $ >41{}^\circ $
• B. $ <41{}^\circ $
• C. $ =41{}^\circ $
• D. $ \ge 41{}^\circ $

Answer 1

Answer: (B)

Given $ A=60{}^\circ $ $ e=46{}^\circ \Rightarrow i=55{}^\circ $ Since, $ i+e=A+\delta $ $ \Rightarrow $ $ 55+46=60+\delta $ $ \Rightarrow $ $ \delta =41{}^\circ $ Practically the angle of minimum deviation for a glass prism is less than $ 40{}^\circ $ (about $ 38{}^\circ $ or $ 39{}^\circ $ ) So, $ \delta <41{}^\circ $