Question

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:

• A. 6m
• B. 3 m
• C. 4m
• D. 2 m

Answer 1

Answer: (A)

6m

Answer 2

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