Question

What should be the angle between two plane mirrors, so that whatever be the angle of incidence, the incident ray and the reflected ray from the two mirrors be parallel to each other?

• A. ${60}^{\circ }$
• B. ${90}^{\circ }$
• C. ${120}^{\circ }$
• D. ${45}^{\circ }$

Answer 1

The correct option is B ${90}^{\circ }$

As incident ray and reflected ray are parallel to each other, it means deviation
$\delta ={180}^{\circ }$
$\because \delta ={360}^{\circ }-2\theta$
$\therefore {180}^{\circ }={360}^{\circ }-2\theta$
$\Rightarrow \theta ={90}^{\circ }$