A luminous point is kept inside tank of water at a depth of 10 m below the surface. $\mu$ for water is $\large\frac{4}{3}$ . What must be the minimum diameter of card placed on the surface of water so that it cuts off all the light coming out.

Question

$(a)\;60\;\pi \\ (b)\;25\;\pi \\ (c)\;180\;\pi \\ (d)\;90\;\pi$