The refractive index of water is 
. This means that the speed of the light in the water is:
The relationship between frequency f and wavelength
of a wave is given by:
where v is the speed of the wave in the medium. The frequency of the light does not change when it moves from one medium to the other one, so we can compute the ratio between the wavelength of the light in water
to that in air as
where v is the speed of light in water and c is the speed of light in air. Re-arranging this formula and by using
, we find
which is the wavelength of light in water.
The relationship between frequency f and wavelength
where v is the speed of the wave in the medium. The frequency of the light does not change when it moves from one medium to the other one, so we can compute the ratio between the wavelength of the light in water
where v is the speed of light in water and c is the speed of light in air. Re-arranging this formula and by using
which is the wavelength of light in water.