The wavelength of light in a medium is λ = λ0/μ, where λ is the wavelength in vacuum A beam of red light (λ0 = 720 nm) enters into water. The wavelength in water is λ = λ0/μ = 540 nm, to a person under water does this light appear green?
When light passes through mediums of different refractive indices, its wavelength and velocity changes by a factor of 
. So the frequency(
) remains constant.
As both wavelength and frequency change by the same amount, the frequency remains the same. So, we would see the same color, that is red, even in water.
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