# Why does light not refract when incidented perpendicularly?

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

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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