Question

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

• A. $\text{PQ}$ is horizontal.
• B. $\text{QR}$ is horizontal.
• C. $\text{RS}$ is horizontal.
• D. $\text{QR}$ is vertical.

Answer 1

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

For minimum angle of deviation,
Angle of incidence $\left(i\right)=$Angle of emergence $\left(e\right)$



From ray diagram of prism, we get
$r_1=r_2$

So, $\text{QR}$ should travel through the prism symmetrically.

Hence, $\text{QR}$ is parallel to the base of the prism, which is horizontal.

Why this Question : This question checks the basic concept of the condition of minimum deviation for a prism.