Using the relation \\dfrac{v_{1}}{v_{2}}=\\dfrac{\\mu _{2}}{\\mu_{1}} where v_1 and v_2 are the respective velocity of light in air and water and \\mu_1 and \\mu_2 are the respective refractive index of air and water.Given : \\mu_2 =\\dfrac{4}{3} and \\mu_1 = 1 Time taken by light to travel a distance x in air 9\ imes 10^{-6} \\ s and that in water be t . \\Rightarrow \\dfrac{\\dfrac{x}{9\ imes 10^{-6}}}{\\dfrac{x}{t}}=\\dfrac{\\dfrac{4}{3}}{1} Or \\dfrac{t}{9\ imes 10^{-6}}=\\dfrac{4}{3} \\Rightarrow t=12\\ \\mu sec