Question

A ray of light travelling through rarer medium is incident at very small angle ${}^{\prime }{i}^{\prime }$ on a glass slab and after refraction its velocity is reduced by 20%. The angle of deviation is :

• A. $\frac{i}{8}$
• B. $\frac{i}{5}$
• C. $\frac{i}{2}$
• D. $\frac{4i}{5}$

Answer 1

The correct option is B $\frac{i}{5}$

Let the speed of light in rarer medium be $c$.

Speed of light in denser medium $v=0.8c$

Using $v\sin i=c\sin r$

As $i$ and $r$ are very small angles, thus $\sin i\approx i$ and $\sin r\approx r$

$\therefore$ $vi=cr$

Or $\left(0.8c\right)i=cr$

$⟹$ $r=\frac{4}{5}i$

Deviation $\delta =i-r$

Or $\delta =i-\frac{4}{5}i=\frac{i}{5}$