Can you answer this?

Solution

 
We need to block the light rays using the disc till total internal reflection takes place.
 
As shown in figure, the direction of light ray at the edge of disc is parallel to light surface, when the incident ray makes critical angle
After the edge, light rays will be reflected due to total internal reflection and the whole radiation from light source is blocked
 
Hence the angle i shown in figure should be critical angle. 
For critical angle we have,  sin i = 1/μ = 3/5
 Now using trignometry, we can find
$$\begin{gathered} Perpendicular = \sqrt{Hypotnus{e}^2-Bas{e}^2} \\ Perpendicular = \sqrt{5^2-3^2} =\sqrt{25-9 } =\sqrt{16} =4 \end{gathered}$$
hence tan i = 3/4 ..............(1)
 
From figure, we see tan i = r/4  ...............(2)
 
hence from (1) and (2),  the required radius = 3 m
So diameter = 2r = 6 m.