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Point source of light is placed 4 m belo...

Point source of light is placed 4 m below water surface in the medium (water) having refractive index `5/3`. A disc is placed on the water surface such that it blocks the light coming out of water completely so minimum diameter of disc = ...........

A

9

B

6

C

4

D

3

Text Solution

Verified by Experts

The correct Answer is:
B
`n=(1)/(sinC)`

`thereforen=((r^2+h^2)^(1/2))/(r)`
`(5/3)^2xxr^2=r^2+h^2`
`therefore(25)/(9)xxr^2=r^2+h^2`
`therefore25r^2=9r^2+9h^2`
`therefore 16r^2=9h^2`
`therefore 4r=3h`
`thereforer=(3h)/(4)`
`therefore r=(3xx4)/(4)`
`therefore r=3`m
`therefore` Diameter =6m
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