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Question:

A source of light lies on the angle bisector of two plane mirrors inclined at angle θ. The value of θ, so that the light reflected from one mirror does not reach the other mirror will be

θ≥120°

θ≤120°

None of the above

θ>90°

Solution:

The most extreme ray that would interfere with another mirror is the ray which will be directed towards the line of angle bisector. This line will make an angle of 90 - θ/2. Now the reflected ray must make some positive angle with the second mirror in order not to interfere with the second mirror. So the governing equation is:
θ/2 - (90 - θ/2) > 0
Above equation implies:
θ > 90