Question

A horizontal ray of light passes through a prism of $\mu =1.5$ whose apex angle is $4^{\circ }$ and then strikes a vertical mirror M as shown. For the ray to become horizontal, either just after reflection from the mirror or finally, the mirror must be rotated through an angle of

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

Answer 1

Answer: (A), (D)

The correct options are
A

$2^{\circ }$

D

$1^{\circ }$

$\delta =(\mu -1)A=(1.5-1)\left(4\right)=2^{\circ }i=\delta =2^{\circ }$

If we rotates the mirror $2^{\circ }$, clockwise ray of light will fall normally and it will retrace its path. If mirror is rotated $1^{\circ }$ clockwise, it will not retrace path but becomes horizontal.