# A point source of light is placed 4m below the surface of a liquid of refractive index 5//3. The minimum diameter of a disc, which should be place

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A point source of light is placed 4m below the surface of a liquid of refractive index 5//3. The minimum diameter of a disc, which should be placed over the source, on the surface of the liquid to cut off all light out of water, is
A. oo
B. 6m
C. 4m
D. 3m

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R=(h)/(sqrt(mu^2-1))=(4)/(sqrt((5/3)^2-1))=(4xx3)/(4)=3m
Diameter d=2R=6m