Question

Choose the correct option for the given assertion and reason.
Assertion: Two plane mirrors kept at right angle deviate any ray of light by ${180}^{\circ }$ after two successive reflections.
Reason: The above condition is satisfied only for angle of incidence, $i={45}^{\circ }$.

• A. Assertion and reason both are correct and reason is the correct explanation of assertion.
• B. Assertion and reason both are correct and reason is not a correct explanation of assertion.
• C. Assertion is correct, but reason is not correct.
• D. Assertion is not correct, but reason is correct.

Answer 1

The correct option is C Assertion is correct, but reason is not correct.


Let the angle of incidence be $i$.
After $1$${}^{\text{st}}$ reflection, angle of deviation,
${\delta }_1={180}^{\circ }-2i$
(Anticlockwise)
After $2$${}^{\text{nd}}$ reflection, angle of deviation,
${\delta }_2={180}^{\circ }-2({90}^{\circ }-i)=2i$
(Anticlockwise)
So, total deviation produced,
$\delta ={\delta }_1+{\delta }_2={180}^{\circ }-2i+2i={180}^{\circ }$ (Anticlockwise)
Hence, Assertion is correct, but reason is not correct.