Question

A horizontal ray of light passes through a prism of index of refraction $\mu =1.50$ & apex angle $4^{\circ }$ and then strikes a vertical mirror, as shown in the figure. Through what angle must the mirror be rotated if after reflection the ray is to be horizontal?

• A. $4^{\circ }$
• B. $3^{\circ }$
• C. $2^{\circ }$
• D. $1^{\circ }$

Answer 1

The correct option is D $1^{\circ }$


As this is the case of thin prism, the minimum angle of deviation,
${\delta }_{min}=(\mu -1)A$
$=(1.5-1)4^{\circ }$
$=2^{\circ }$

Now, let the mirror be rotated by an angle $\theta$ in clockwise direction from the diagram . Thus we can say, $2\theta =\delta$
$\therefore 2\theta =2^{\circ }$
$\Rightarrow \theta =1^{\circ }$

Hence, option (d) is the correct answer.

$$\begin{array}{c}\text{Why this question:}\\ \text{To understand the concept of minimum devitation \&}\\ \text{revision of effect of rotation of plane mirror.}\end{array}$$