Question

If incident ray is normal to one of the surfaces of equilateral prism and suffers total internal reflection on second, then deviation caused by prism is

• A. $0^o$
• B. ${60}^o$
• C. ${90}^o$
• D. ${180}^o$

Answer 1

The correct option is B ${60}^o$

As the incident ray is normal to the surface , therefore it will enter (at point P) in prism without deviation .

In $△BPQ$ ,

$\angle P={90}^o$ ,

$\angle B={60}^o$ ( as riangle ABC is quadrilateral) ,

hence $\angle Q=180-(90+60)={30}^o$ ,

Now NN' is a normal at point Q ,

therefore , $\angle PQ{N}^{\prime }=90-30={60}^o$ ,

by $\angle i=\angle r$ at Q ,

$\angle PQR={60}^o$ ,

hence , angle of deviation

$\angle RQS=180-(60+60)={60}^o$