### NEET Solved Paper 2018 Question 19

##### Question: The refractive index of the material of a prism is $ \sqrt{2} $ and the angle of the prism is $ \text{30 }{}^\circ $ . One of the two refracting surfaces of the prism is made a mirror inwards, by silver coating. A beam of monochromatic light entering the prism from the other face will retrace its path (after reflection from the silvered surface) if its angle of incidence on the prism is [NEET - 2018]

#### Options:

A) $ \text{30 }{}^\circ $

B) $ 45{}^\circ $

C) $ 60{}^\circ $

D) Zero

## Show Answer

#### Answer:

Correct Answer: B

#### Solution:

For retracing its path, light ray should be normally incident on silvered face. Applying Snell’s law at M, $ \frac{\sin i}{\sin 30{}^\circ }=\frac{\sqrt{2}}{1} $

$ \Rightarrow \sin i=\sqrt{2}\times \frac{1}{2} $ $ \sin i=\frac{1}{\sqrt{2}}i.e.i=45{}^\circ $