Red light of wavelength 640 nm in air enters a pool of water of refractive index 1.33…

Step 1. The frequency. $f = \dfrac{c}{\lambda_0} = \dfrac{3.00 \times 10^{8}\ \text{m/s}}{640 \times 10^{-9}\ \text{m}} = 4.69 \times 10^{14}\ \text{Hz}$.

Step 2. The speed. $v = \dfrac{c}{n} = \dfrac{3.00 \times 10^{8}\ \text{m/s}}{1.33} = 2.26 \times 10^{8}\ \text{m/s}$.

Step 3. The wavelength. $\lambda = \dfrac{\lambda_0}{n} = \dfrac{640\ \text{nm}}{1.33} = 481\ \text{nm}$.