Question

An Engineer intends to deviate a light ray from it's part by 120° through reflection from a plane mirror. Calculate the angle of incidence.

Answer 1

The angle of deviation can be written as follows:

$$\delta=180^{\circ}-(i+r),$$

here, $i$ is the angle of incidence, $r$ is the angle of reflection.

According to the law of reflection, the angle of reflection is equal to the angle of incidence.Therefore, we can write:

$$\delta=180^{\circ}-2i,$$ $$i=\dfrac{180^{\circ}-\delta}{2}=\dfrac{180^{\circ}-120^{\circ}}{2}=30^{\circ}.$$