Blue Light

We know that for a light wave: $v = f \lambda$ where $f$ is the frequency and $\lambda$ is the wavelength. Now, as light passes from one medium to another, its frequency remains unchanged since the frequency of the wave depends on the light source itself. What does change is the wavelength. Let the speed of blue light in air and its corresponding wavelength be $v_1$ and $\lambda_1$ respectively. Let the speed of blue light in water and its corresponding wavelength be $v_2$ and $\lambda_2$ respectively. Then:

$v_1 = f\lambda_1$ $v_2 = f \lambda_2$

Dividing both equations and rearranging:

$\implies v_2 = \frac{v_1 \lambda_2}{\lambda_1} \approx 2.2 \times 10^{8} \ \mathrm{m/s}$