Question

A light ray is incident on plane mirror at an angle ${30}^{\circ }$ with the horizontal. At what angle with horizontal must a plane mirror be placed in its path so. that it becomes vertically upwards after reflection ?

• A. ${30}^0$
• B. ${45}^0$
• C. ${60}^0$
• D. ${90}^0$

Answer 1

The correct option is A ${30}^0$

We know that the angle of incidence is equal to the angle of reflection. It is given that the incident ray makes an angle ${30}^{\circ }$ with the plane and hence the reflected ray also makes an angle ${30}^{\circ }$ with the plane.

Hence the ray will be reflected in a vertical direction if the mirror is to be rotated by an angle of ${60}^{\circ }$.

$\therefore \theta ={60}^{\circ }$

The mirror should be rotated by an angle $\frac{\theta }{2}=\frac{{60}^{\circ }}{2}={30}^{\circ }$

The correct answer is option (A).