When a photon goes say from air to water, it slows down according to; n=c/v where n is refractive index of water c is velocity of light in air v is velocity of light in water It is stated that the frequency of the photon doesn't change when the photon enters the water. Only the wavelength changes. I cannot find a proof for this. Does one exist? Or is it just based on the conservation of energy E=hf ? Is it that the frequency can't change so the wavelength must change?