Question

A plane mirror makes an angle of ${30}^{\circ }$ with the horizontal. If a ray strikes the mirror vertically as shown in the figure, find the angle of deviation of reflected ray from the incident ray.

• A. ${120}^{\circ }$ clockwise
• B. ${30}^{\circ }$ clockwise
• C. ${30}^{\circ }$ counter-clockwise
• D. ${120}^{\circ }$ counter-clockwise

Answer 1

The correct option is D ${120}^{\circ }$ counter-clockwise


Angle of deviation is the angle between the incident and reflected rays.
In the figure, angle of deviation is marked as $\delta$.

We can deduce that,

$\delta +{30}^{\circ }+{30}^{\circ }={180}^{\circ }$

$\Rightarrow \delta ={180}^{\circ }-{60}^{\circ }={120}^{\circ }\left(CCW\right)$

Hence, option (d) is the correct answer.

$\begin{array}{c}\text{Why this Question}?\\ \text{This question helps you to check your understanding of angle of deviation}\\ \text{Note :- Direction of deviation is very important.}\\ \text{Let's try :- For the same given question, instead of right inclined mirror choose a left inclined mirror}\\ \text{and find the angle of deviation.}\end{array}$