Question

In figure, a ray of light $PQ$ is incident normally on the hypotenuse of an isosceles right angle prism $ABC$. What is the angle of deviation of the ray $PQ$?

• A. ${90}^o$
• B. ${45}^o$
• C. ${180}^o$
• D. ${30}^o$

Answer 1

Answer: (C)

${180}^o$

The incident ray PQ enters the prism undeviated and strikes the surface AB with an angle of incidence of ${45}^o$ which is greater than the critical angle for glass i.e. ${42}^o$. Hence it undergoes total internal reflection and the reflected ray will strike the surface BC with an angle of ${45}^o$ and undergo total internal reflection again. This reflected ray will strike the surface AC normally and travel straight. Thus the total deviation is ${180}^o$.