Question

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

• A. ${50}^0$
• B. ${45}^0$
• C. ${30}^0$
• D. ${55}^0$

Answer 1

The correct option is A ${50}^0$

$\theta =b$

Angle of incidence =b=reflection angle

Look at angles in the triangle formed by the reflected ray and 2 mirrors.

Angle of incidence on 2nd mirror,

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

Angle between reflected ray and 2nd mirror$b+20$

As it is parallel to 1st mirror, $b+20=70$

$b={50}^{\circ }$