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Originally Posted by GMontag
Both views (light as waves and light as particles) are valid and useful. The wave explanation for light slowing down in a medium is that the speed of light is determined by two constants, the permittivity and the permeabilitity of the medium. Those constants change from medium to medium. The light as particle explanation for the slow down is that the photons are all actually still travelling at c, but whenever they run into an atom, they get absorbed and reemitted, which takes time. On average then, the speed of light slows down.
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Just to add to this, the permittivity and permeability are not just empirical constants that happen to be different for every material. It is possible to derive them (at least in principle) using just the permittivity and permeability of a vacuum, along with knowledge of the material's internal structure. Light travels through a medium at the same speed as it does in a vacuum, but its interactions with the atoms inside of it cause them to give off their own fields. This secondary field combines with the main one in such a way that it looks as if light has actually slowed down on macroscopic scales (only the sum of the waves is measurable).