How Does Light Travel Through Glass?

I’ve mentioned before that I’m answering the occasional question over at the Physics Stack Exchange site, a crowd-sourced physics Q&A. When I’m particularly pleased with a question and answer, I’ll be promoting them over here like, well, now. Yesterday, somebody posted this question:

Consider a single photon (λ=532 nm) traveling through a plate of perfect glass with a refractive index n=1.5. We know that it does not change its direction or other characteristics in any particular way and propagating 1 cm through such glass is equivalent to 1.5 cm of vacuum. Apparently, the photon interacts with glass, but what is the physical nature of this interaction?

I didn’t have a ready answer for this one, but I’m pretty happy with what I came up with on the spot, so I’ll expand on it a little bit here. I think it’s an interesting question not only because the issues are a little bit subtle, but because it also shows the importance of understanding classical models as well as quantum ones. The key to understanding what’s going on here in the quantum scenario is to recognize that the end result is the same as in the classical case, and adapt the classical method accordingly.

So, how do you explain this classically, that is, in a model where light is strictly a wave, and does not have particle character? The answer is, basically, Huygens’s Principle.

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