11. A point source of light is placed 4 m 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 ?

11. A point source of light is placed 4 m 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) ∞

(b) 6 m

(c) 4 m

(d) 3 m

Answer :  (b) 6 m

\( \text{Critical Angle } i_c : \sin i_c = \frac{1}{\mu} = \frac{1}{\frac{4}{3}} = \frac{3}{4} \)

\( \tan i_c = \frac{x}{4} \)

\( \tan i_c \approx \sin i_c = \frac{3}{4} \)

\( \frac{3}{4} = \frac{x}{4} \Rightarrow x = 3 \)

\( \text{Diameter} = 2x = 2 \times 3 = 6\, \text{m} \)