Given wavelength of sodium light in air is

(a)Therefore, the frequency is .

(b) Wavelength of light in a medium of refractive index is given by .

Therefore, the wavelength of sodium light in water is

(c)Frequency of light doesn’t change due to a change in medium as both the speed of light and the wavelength of light are reduced by the same amount(). So the frequency is .

(d)The speed of light in a medium is given by