Question

A ray of light is incident on a plane mirror at an angle $\theta$. If the angle between the incident and reflected rays is ${80}^{\circ }$, what is the value of $\theta$ ?

• A. ${40}^{\circ }$
• B. ${50}^{\circ }$
• C. ${45}^{\circ }$
• D. ${55}^{\circ }$

Answer 1

The correct option is B ${50}^{\circ }$


given : $\angle i+\angle r={80}^0$
Solution: $\angle i+\angle r={80}^0$ $\to \left(1\right)$
and for a plane mirror
$\angle i=\angle r$ $\to \left(2\right)$
then from (1) and (2)
$2\angle i={80}^0$
$\angle i={40}^0$
Hence

Hence from the figure $\theta +{40}^0={90}^0$
Hence $\theta ={50}^0$