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  4. A point source of light is placed 4 m below the surface of water of refractive index is 5/3. The minimum diameter of a disc which should be placed over the source on the surface of water to cut-off all light coming out of water is:

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Q. A point source of light is placed 4 m below the surface of water of refractive index is 5/3. The minimum diameter of a disc which should be placed over the source on the surface of water to cut-off all light coming out of water is:

JIPMERJIPMER 2001Ray Optics and Optical Instruments

Solution:

Given, $ \sin \,i=\frac{1}{\mu }=\frac{3}{5}, $ so $ \tan i=\frac{3}{4}=\frac{r}{4} $ it gives r = 3 m so, the diameter = 6 m