A ray of light strikes a plane mirror PQ at an angle of incidence of ${30}^{\circ }$, is reflected from the plane mirror and then strikes a second plane mirror QR placed at right angles to the first mirror. The angle of reflection at the second mirror is : (a) ${30}^{\circ }$ (b) ${45}^{\circ }$ (c) ${60}^{\circ }$ (d) ${90}^{\circ }$ Draw a ray-diagram to illustrate your answer.

angle of incidence = angle of reflection. ----1

we know that an incident ray which strikes the first mirror I.e PQ at the angle of incidence 30°

also, by 1 we get that angle of reflection for the first case would be 30°

now the same reflected ray again in the form of incident ray makes an angle of incidence of 30° and gets reflected by the angle of 30°.

total reflection would have been: 30° + 30°i.e 60°

angle of reflection at the second mirror is 60°.