Question

An equilateral prism is placed on a horizontal surface. A ray $\text{PQ}$ is incident onto it. For minimum deviation

• A. $\text{PQ}$ should be horizontal.
• B. $\text{QR}$ should be horizontal
• C. $\text{RS}$ should be horizontal
• D. Any ray can be horizontal

Answer 1

The correct option is B $\text{QR}$ should be horizontal


For minimum deviation,
$i=e\text{and}r=r^{\prime }$

And we know that deviation $\delta$ in prism,

$\delta =(i+e)-A$

${\delta }_{min}=2i-A(\because i=e)$

Here, $r=r^{\prime }$ and we know that

$r+r^{\prime }=A$

$\Rightarrow r+r=A$

$\therefore r=A/2\Rightarrow$ which is possible in the case of symmetry.

$\therefore \text{QR}$ must be horizontal.

Hence, option $\left(b\right)$ is correct.