Quote:
Originally Posted by Ken G
And yes, the "speed in a medium" depends on how you treat the medium. If you treat it as one continuous thing, then you can get a slower speed. If you treat it as a discrete bunch of atoms, then there is a sense to which the speed is c in the vacuum "between atoms", but the combined effect of all the atoms slows the net progress of the signal.
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Yes, this is what I'm getting at - photons always travel at c. But if you measure the travel time of light through the medium, it will be less than c, because of the delay caused by all those atoms absorbing and re-emitting the photons. The photon that you measured exiting the medium is not the same one that entered on the upstream end.
Assuming this is correct, I don't understand why it's said that the speed of light varies depending on the medium. In the post that I'm referring to, the poster seemed to be saying that the photons were actually traveling at a slower speed - I believe that is incorrect, and this is why I want to be sure that I understand exactly what is happening.
Now, to further complicate matters, I'll add that I was under the impression that a transparent material passes light because it (ideally) contains no atoms with electrons having energy levels matching those of photons of the wavelength at which the material is transparent, so no photons are absorbed. If this is the case, would light travel more slowly through a perfectly transparent medium?