Question

Light falls from glass $(\mu =1.5)$ to air. Find the angle of incidence for which the angle of deviation is ${90}^o$.

Answer 1

Since, $\mu =1.5$, Critial angle $={\sin }^{-1}\left(1/\mu \right)={\sin }^{-1}\left(1/1.5\right)={41.8}^{\circ }$

We know, the maximum attainable deviation in refraction is $({90}^{\circ }-{41.8}^{\circ })={47.2}^{\circ }$

So, in this case, total internal reflection must have taken place.

In reflection,

Deviation $={180}^{\circ }-2i={90}^{\circ }\Rightarrow 2i={90}^{\circ }\Rightarrow i={45}^{\circ }$.