Q. A point source of light is placed 4 m below the surface of water of refractive index 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
A) Infinite
B) 6 m
C) 4 m
D) 3m
Explanation
(b) Let $S$ be the light source. If light falls on the surface at critical angle $C,$ it grazes along the surface as shown.
$\sin{C = \frac{1}{n} = \frac{1}{5}}$
From $\mathrm{\Delta}QSR,$ we have
$\tan{C = \frac{QR}{QS} = \frac{r}{4}}$
$\Rightarrow \frac{3}{4} = \frac{r}{4}$
$\Rightarrow r = 3$
Hence, diameter $= 2r = 2 \times 3 = 6\ m$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.