Question

A point source of light is placed $4m$ 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 is :

• A. $6m$
• B. $3m$
• C. $4m$
• D. $2m$

Answer 1

The correct option is A $6m$

Light from the source goes in all directions, forming an angle of incidence to the surface of water.

For angles greater than a critical angle, the light rays reflect back into the water.

The critical angle $={\theta }_c=si{n}^{-1}\frac{1}{\mu }=si{n}^{-1}\frac{3}{5}$

Thus, the radius of the disc required $=h\times tan{\theta }_c=4\times \frac{3}{4}m=3m$

Hence, the diameter of the disc $=6m$