An isotropic point source is placed at a depth h below the water surface. A floating opaque disc is placed on the surface of water so that the source is not visible from the surface. What is the minimum radius of the disc? Take refractive index of `"water"=mu`.

Explanation

As shown in figure, light from the source will not emerge out of water, if i `gttheta_(C)`.

Therefore minimum radius R corresponds to, i=`theta_(C)`
In `DeltaSAB`

`R/h=tan theta_(C)`
`therefore R=h tan theta_(C)`
or `R=h/(sqrt(mu^(2)-1))`