# Wavelength of a colour of light for air is $6000\;A^{\circ}$ and in water the wavelength is $4500\;A^{\circ}$. What is the speed of light in water.

$(a)\;5.0 \times 10^{14}m/s \\ (b)\;2.25 \times 10^8 m/s \\ (c)\;4.0 \times 10^8 m/s \\ (d)\;1.5 \times 10^{14}$

Velocity $\times$ wavelength = constant
$\large\frac{v_1}{v_2} =\frac{\lambda _1}{\lambda_2}$
$\large\frac{v_2}{3 \times 10^8 }=\frac{4500}{6000}$
$\qquad= 2.25 \times 10^8 m/s$
Hence b is the correct answer.

edited Jul 14, 2014