# Why frequency does not change when light passes through the denser medium? [duplicate]

as far as I noticed always people in physics have a predefined assumption that frequency is constant. whereas we know that the c is the outcom of product of wavelength and frequency. we have different wavelength and frequency iv whit light. the energy depends on both of them wavelength and frequency. if you consider the speed of light as a global constant parameter then both attribute of that must have that properties. but we know when light passes through a glass for example; it loses energy as we can measure the changes in temperature of glass that is increased. then I think the change of speed of light is because of interaction between light as an electromagnetic wave and fields inside the material would occure and frequency and wavelength both would change by 1 divided on square root of $$n$$ or refracting index. by the way we lose the speed but when the light enters the air again it behaves as before then we have the same speed. you van not measure the speed at the boundary condition as speed needs distance and time to be measured. then why we must suppose frequency is constant?

• Related or duplicate: physics.stackexchange.com/q/21336, physics.stackexchange.com/q/22385, and physics.stackexchange.com/q/263288 is framed in the same terms as here. Jun 22, 2019 at 17:44
• "always people in physics have a predefined assumption about frequency is constant" This should be rephrased. Jun 22, 2019 at 18:13
• hi. I had read all of them. again and again the assumption is repeated that frequency is fixed. if you start that c is constant and if you accept that the wavelength and frequency are related by a constant then each of them can change. i can not imagin the wavelength without frequency. furthermore wave length is very important more than frequency for color perception as the size of the cone cells and rods are rlated to their spectrum sensitivity. at the end suppose wavelength is constant then you can do all those arguments for proving that wavelength is constant. Jun 23, 2019 at 11:42
• @A.H.Kaidan, frequency is determined by the source of the wave. When light hits a dense medium and slows down, the wavelength shortens, the frequency remains constant, and the equation $v=f\lambda$ still applies. This means that $v$ and $\lambda$ change by the same percentage, such that frequency remains constant. Jun 23, 2019 at 15:36