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

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

Answer: According to the laws of reflection, 
angle of incidence = angle of reflection.

As given, ∠i + ∠r = 90
It implies ∠i = ∠r = 90/2 = 45 degrees. Angle of incidence = 45 degrees.
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