Question

Two plane mirrors inclined at ${70}^0$ . A incident on one mirror at an angle $\theta$ after reflection falls on the second mirror and reflected from there parallel to the first mirror then $\theta$ is :

• A. ${50}^{\circ }$
• B. ${45}^{\circ }$
• C. ${30}^{\circ }$
• D. ${55}^{\circ }$

Answer 1

Answer: (A)

${50}^{\circ }$

Angle incidence on the $2nd$ mirror.

$=90-[180-70-(90-b\left)\right]=70-b$

Angle between the reflected ray and the $2nd$ mirror.

$=b+20$

As it is parallel to $1st$ mirror

$\begin{array}{c}\therefore b+20=70\\ \therefore b=50\end{array}$

Hence, Option $A$ is correct.