Question

A small source of light is 4m below the surface of a liquid of refractive index 5/3. In order to cut off all the light coming out of liquid surface, minimum diameter of the disc placed on the surface of liquid is

• A. 3m
• B. 4m
• C. 6m
• D. $\infty$

Answer 1

Answer: (C)

6m

In order to not let the light from the source, it should be totally internally reflected.
i.e the $i>c$

Now, $sini>sinc$

$\mu =\frac{5}{3}$

Thus, sin c = $\frac{3}{5}$

This gives the value of tan c = $\frac{3}{4}$

Now, if r be the radius of the disc, then

$tanc=\frac{r}{h}$

$\frac{3}{4}=\frac{r}{4}$

Thus, $r=3$

Or, $d=6$