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A small angled prism of apex angle A=4^@...

A small angled prism of apex angle `A=4^@` and refractive index `mu=1.5` is placed in front of a vertical plane mirror as shown in figure. If the mirror is rotated through an angle `theta,` then the light ray becomes horizontal either after the mirror or after second time passing from the prism in opposite direction. The value `theta` is

A

`1^@`

B

`2^@`

C

`4^@`

D

Not possible

Text Solution

Verified by Experts

The correct Answer is:
A, B
Deviation , `delta=(mu-1)A=(1.5-1)4^@=2^@`

To rotate ray MP by `2^@` (to make it parallel to
MN) we will have to rotate the mirror only
by `1^@.`
Without prism, ray of light was falling normal
to plane mirror. So, ray of light was retracing
its path prism has deviated it `2^@.` So, if we
rotate the mirror by `2^@,` ray of light further falls
normal to plan mirror and retraces its path.
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