Two mirrors (A) and (B) are at right angle to each other. If a ray of light strikes mirror (A) at 20 degree to the normal and then strikes mirror (B), find the angle of reflection of the ray emerging from mirror (B).
70 degree
By the law of reflection, the ray leaving mirror (A) also makes an angle of 20 degree with the normal to mirror (A).
Because mirror (A) and mirror (B) are perpendicular to each other, their normals are also perpendicular to each other, so the two normals together with the ray path between the mirrors form a triangle in which one angle is 90 degree.
In this triangle, the angle the ray makes with the normal to mirror (A) is 20 degree, so the angle it makes with the surface of mirror (A) is (90 degree - 20 degree) = 70 degree; since mirror (A) and mirror (B) are perpendicular, this same ray makes an angle of (90 degree - 70 degree) = 20 degree with the surface of mirror (B), which means it makes an angle of (90 degree - 20 degree) = 70 degree with the normal to mirror (B).
By the law of reflection, the angle of reflection at mirror (B) is equal to its angle of incidence, that is, 70 degree.
Hence, the angle of reflection of the ray emerging from mirror (B) is 70 degree.
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