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Question

A small source of light is 4m below the surface of a liquid of refractive index 5/3. In order to cut off all the light coming out of liquid surface, minimum diameter of the disc placed on the surface of liquid is

A
3m
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B
4m
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C
6m
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D
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Solution

The correct option is B 6m
In order to not let the light from the source, it should be totally internally reflected.
i.e the i>c

Now, sini>sinc

μ=53

Thus, sin c = 35

This gives the value of tan c = 34

Now, if r be the radius of the disc, then

tanc=rh

34=r4

Thus, r=3

Or, d=6

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