21. A small coin is resting on the bottom of a beaker filled with liquid. A ray of light from the coin travels upto the surface of the liquid and moves along its surface. How fast is the light travelling in the liquid ?

21. A small coin is resting on the bottom of a beaker filled with liquid. A ray of light from the coin travels upto the surface of the liquid and moves along its surface. How fast is the light travelling in the liquid ?

(a) \( 2.4 \times 10^8 \, \text{m/s} \)

(b) \( 3.0 \times 10^8 \, \text{m/s} \)

(c) \( 1.2 \times 10^8 \, \text{m/s} \)

(d) \( 1.8 \times 10^8 \, \text{m/s} \)

Answer :  (d) \( 1.8 \times 10^8 \, \text{m/s} \)

\( \sin i_c = \frac{3}{5} = \frac{1}{\mu} \)

\( \mu = \frac{c}{v} \)

\( \Rightarrow v = \frac{c}{\mu} = 3 \times 10^8 \times \frac{3}{5} \)

\( = 1.8 \times 10^8 \, \text{m/s} \)