• # question_answer In the figure ABC is the cross section of a right angled prism and BCDE is the cross section of a glass slab. The value of $\theta$ so that light incident normally on the face AB does not cross the face BC is $(given\,{{\sin }^{-1}}(3/5)=37{}^\circ )$ A) $\theta \le 37{}^\circ$              B) $\theta <37{}^\circ$ C) $\theta \le 53{}^\circ$  D) $\theta <53{}^\circ$

Solution :

[B] $A=90-\theta$ ${{r}_{2}}=A=90-\theta >{{\theta }_{C}}$ $\cos \theta >\sin {{\theta }_{C}}=\frac{6/5}{3/2}=\frac{4}{5}$ $\Rightarrow \theta <{{\cos }^{-1}}\frac{4}{5}=37{}^\circ .$

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