Question

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`

Answer 1

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