What is the intuition for why the frequency of light does not change as it passes from a less dense medium to a denser one (or vice versa)? Classically, if we treat light in terms of waves, then intuitively, is the reason why the frequency does not change because it is determined by the emitting source, i.e. the source emit waves of light at a particular frequency. As such, there will be a certain number of wave crests crossing the boundary between the two media per unit of time, and since the boundary between the two media doesn't destroy wave crests, the number of wave crests arriving at one side of the boundary per unit time must equal the number of crests leaving the boundary on the other side, per unit time. Hence the frequency is unchanged in the transmission from one medium to another. Could one also argue that the frequency must remain constant on the grounds of energy conservation at the boundary. Since the refraction of light, as it propagates from one medium into another, is not dissipative this requires that the energy of a light wave must be the same on either side of the boundary, and since its energy is given by ##E=h\nu##, this requires that the frequency, ##\nu## of the wave must remain constant.