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I had read that light does slow down in glass because photons interact with atoms in glass. They are absorbed and re-emitted and during this phenomenon it's speed decreases. See also this and this Phys.SE post and links therein.

But then why does it bend? It has confused me a lot! Furthermore i have very little knowledge of Optics or Physics as a whole.

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The title doesn't correspond to the content of the question. The answer to the question posed in the title is that it's simply a matter of symmetry. –  Ben Crowell Jul 6 '13 at 0:47

2 Answers 2

A good question, and one that confused me for a long time.

In the absense of matter the light wave is massless. However in the presence of polarisable matter the light interacts with the matter and forms a composite system. Now you have a single wavefunction that describes the combined system and you can no longer separate it into a light bit and matter bit. This composite system propagates with a velocity of less than $c$.

The composite system of light and matter is called a polariton although I think strictly speaking the term polariton is reserved for strongly interacting systems such as Bose-Einstein condensates, where the speed of light can be reduced to only a few metres per second.

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Fermat's "Least Action Principle"

To understand the reason why this happens, you need to think in terms of Fermat's least action principle. The reason why light refracts is because it "optimises" its path. In other words, light takes the path of minimal time. This is the principle that was stated by Fermat, and if you do the mathematical analysis of it, which is not hard, you can derive Snell's law. In quantum mechanics this is explained by the infinitely many paths light takes, and the way in which the phasors add up to give the most probable path.

When light falls vertically on the interface of two media, the straight through path is the shortest path from time point of view. So there is no need for light to divert its direction. Of course, some of it will be reflected back out. For example if it is air-to-glass, I think about 4% will be reflected and the rest will be transmitted.

As for the question "why light slows down", it has been discussed elsewhere in this forum.

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