Question

Two plane mirrors subtend an angle $\theta$. Light incident on one mirror after reflection from the other mirror returns parallel to the original direction. The angle $\theta$ must be

• A. ${30}^o$
• B. ${45}^o$
• C. ${60}^o$
• D. ${90}^o$

Answer 1

Answer: (D)

${90}^o$

Here
$\delta =360-2\theta$
Light incident on one mirror after reflection from the other mirror returns parallel to the original direction
so
$\delta ={180}^{\circ }$
$360-2\theta =180$
$\theta ={90}^{\circ }$