This is something I really should know but found I was unable to explain it to myself. When a ray of light passes from one medium to another its frequency remains invariant, but it slows down, forcing the wavelength to decrease according to [itex]c = \nu\lambda[/itex]. The frequency of the wave will correspond to the frequency of the elementary charged oscillators responsible for its production. What is it about the frequency of the wave that leaves it unaffected, as opposed to the wavelength which is free to change? This leads me to a related question: is it practically possible to change the frequency of a ray of light, how would one do that anyway?