Question

Two plane mirrors are inclined to each other at an angle $\theta$. A ray of light is reflected first at one mirror and then at the other. The total deviation of the ray is

• A. $2\theta$
• B. $240^{\circ }-2\theta$
• C. $360^{\circ }-2\theta$
• D. $180^{\circ }-\theta$

Answer 1

Answer: (C)

Total deviation $=\left(180^{\circ }-2\alpha \right)+\left(180^{\circ }-2\beta \right)$
$=360^{\circ }-2(\alpha +\beta )$
But, $90^{\circ }-\alpha +90^{\circ }-\beta +\theta =180^{\circ }$
or $\theta =\alpha +\beta$

$\therefore$ Total deviation $=360^{\circ }-2\theta$