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 $=360^{\circ}-2 \theta$