Question

Two plane mirrors are inclined to each other such that a ray of light incident on the first mirror $\left(M_1\right)$ and parallel to the second mirror $\left(M_2\right)$ is finally reflected from the second mirror $\left(M_2\right)$ parallel to the first mirror $\left(M_1\right)$. The angle between the two mirrors will be :

• A. ${90}^o$
• B. ${45}^o$
• C. ${75}^o$
• D. ${60}^o$

Answer 1

Answer: (D)

${60}^o$

Assuming angles between two mirrors be $\theta$ as per geometry,
sum of angles of $\Delta$

$\lambda 1+\lambda 2=360-20=180+\theta$
$3\theta ={180}^o$
$\theta ={60}^o$