In air, sodium light has a wavelength of 589 nm . Determine its...

The frequency of light is a characteristic of the source and remains constant when it travels from one medium to another.

Therefore, the frequency of sodium light in water is the same as its frequency in air.

Using the relation $c = \nu \lambda$, where $c$ is the speed of light in vacuum and $\lambda$ is the wavelength in air:

$\nu = \dfrac{c}{\lambda}$

$\nu = \dfrac{3 \times 10^{8}}{589 \times 10^{-9}}$

$\nu \approx 5.09 \times 10^{14} \text{ Hz}$