What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
45°
By the law of reflection, the angle of incidence equals the angle of reflection, both measured from the normal to the reflecting surface.
The angle between the incident ray and the reflected ray equals the sum of the angle of incidence and the angle of reflection.
Since the two angles are equal, each one is half of the angle between the two rays: 90° ÷ 2 = 45°.
Checking this: if the angle of incidence is 45°, the angle of reflection is also 45°, and together they add up to 90°, matching the given condition.
Hence, the angle of incidence is 45°.
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