What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Here, the angle of reflection is 90°. As we know, according to the laws of reflection that angle of incidence is equal to angle of reflection. Here, the angle between the incident ray and reflected ray is 90°. i.e., ∠i + ∠r = 90° Since, ∠i = ∠r We can write, ∠i + ∠i = 90° ⇒ 2∠i = 90° ⇒ ∠i = 45° Angle of incidence = 45°.