Question

A point source of light is placed at a depth of h below the surface of water of refractive index $\mu$. A floating opaque disc is placed on the surface of water so that light from the source is not visible from the surface. The minimum diameter of the disc is:

• A.
• B.
• C.
• D.

Answer 1

Answer: (A)

Answer 2

: The figure shows incidence from water at critical angle for the limiting case.

Now, sin

So that

From figure,

where r is the radius of disc. Therefore, diameter of the disc is