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Question

Two mirrors are kept at an angle 60 between them as shown in the fig. A ray of light strikes the first mirror and after reflection strikes the second mirror. If the final reflected ray goes parallel to the first mirror, calculate the angle of incidence.


A

10

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B

20

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C

30

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D

40

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Solution

The correct option is C

30


Lets draw the ray diagram as mentioned in the problem.

1=90θ (Angle of incidence = angle of reflection) ...1)
1=3+4 (Alternate opposite angles) ...2)
3=4 (Angle of incidence = angle of reflection) ...3)
From above three we get,
3=45θ2.
3+2=90.
(as BD is the normal)
Hence
2=45+θ2
Now, in triangle, ABO
1+2+60=180
Substituting 1 & 2, we get
90θ+45+θ2=120
From above equation
θ2=15
Or,
θ=30


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