Why does an underwater object appear blue instead of the expected color?

[ehild]

As @jedishrfu pointed out, the color of light depends on the energy of the photons, E=hf, that is, on frequency f. As it is a very high number, and old spectrometers measured light intensity in terms of wavelength, we say that visible light is between 400 and 800 nm of vacuum wavelength, (λ₀=c/f) The frequency of light does not change at refraction. The wavelength changes, but the color does not.