[cbolton]

Is this more than a change of terminology? Is it wrong to say that the electromagnetic oscillation is still there in the medium but coupled to a mechanical oscillation, and the coupled system is called a phonon? (I mean there's still an electromagnetic field in the material, it still gets excited, it's not replaced with a new field that fuses the EM and matter fields). Then the question is by which process exactly is the electromagnetic oscillation slowed down, and is the answer much different from the explanations above? I.e. the moving charges due to the mechanical oscillation add perturbations to the EM field such that the net effect is a slower propagating wave.