Question

A point source of light is placed $4$$m$ below the surface of water of refractive index $\frac{5}{3}$. 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. Infinite
• B. $6m$
• C. $4m$
• D. $3m$

Answer 1

The correct option is B $6m$

$\sin {\theta }_c=\frac{1}{\mu }=\frac{3}{5}$
$\frac{r}{4}=\tan {\theta }_c=\frac{3}{4}$
Radius, $r=3m$
Diameter, $d=6m$