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Question

Use the following information to answer the next question.

Arun has a combination of two plane mirrors, M1 and M2. The mirrors are placed at an angle θ with each other, as shown in the given figure.

A light ray is incident on M1, with the incident angle being 20°.

For what value of , will angle be 30°?


A

25°

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B

45°

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C

80°

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D

90°

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Solution

The correct option is C

80°


The angle of incidence is always equal to the angle of reflection. Hence the given situation can be shown as:

∠PQX = ∠XQS = 20°

Since XQ is the normal to mirror M1, ∠SQR = 90° − 20° = 70°

It is given that ∠TSF = = 30°

Since YS is normal to mirror M2, ∠YST = 90° − 30° = 60°

∴ ∠QSY = ∠YST = 60°

Again, ∠QSR = 90° − ∠YSQ = 90° − 60° = 30°

In triangle ΔRQS:

θ + 70° + 30° = 180°

∴θ = 180° − 100° = 80°


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