I had a question that is probably a bit OT for this group (why does 
light travel slower in glass), but Google found me the answer, in fact a 
*very* good answer:

http://www.feynmanlectures.caltech.edu/I_31.html

Reading some of the other chapters on his page, this guy is obviously 
good at explaining and knows his stuff :-)

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That's quite an interesting presentation. (And I didn't know Feynman's lectures
were available online; thanks for the link.)

Something that has always puzzled me about this topic (once I started thinking
about it!) was an apparent paradox: If light shines on a glass plate in a
vacuum, the light 'apparently' slows down, inside the glass (according to the
glass's index of refraction.) But when it emerges from the other side, it speeds
back up to the speed of light(!) Since the light beam didn't 'gain' any energy
inside the glass, it seemed obvious that the light didn't really 'slow down,'
but that some other explanation was necessary-- which Feynman describes nicely
(if a bit too mathematically, for my poor brain.)

I've also read other explanations of this idea that light slows down inside a
material: simply put, that the photons have to exchange their energy with other
electrons as they pass through, in a cascading fashion from one atom to the
next-- which takes a finite time, from entry to exit. I'm not sure if this idea
is in agreement with Feynman's explanation, though.

By the way, another good source of info is Feynman's (small) book QED: THE
STRANGE STORY OF LIGHT AND MATTER (about quantum electrodynamics.) It's
surprisingly readable(!), without much math at all. Highly recommended.

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> Something that has always puzzled me about this topic (once I started thinking
> about it!) was an apparent paradox: If light shines on a glass plate in a
> vacuum, the light 'apparently' slows down, inside the glass (according to the
> glass's index of refraction.) But when it emerges from the other side, it speeds
> back up to the speed of light(!) Since the light beam didn't 'gain' any energy
> inside the glass, it seemed obvious that the light didn't really 'slow down,'
> but that some other explanation was necessary-- which Feynman describes nicely
> (if a bit too mathematically, for my poor brain.)

Yes, there is quite a lot of maths there, but he (quite rightly!) 
assumes you've studied the previous chapters on EM waves. And he does 
write "This is probably the most complicated thing that we are going to 
do this year"... !!

> I've also read other explanations of this idea that light slows down inside a
> material: simply put, that the photons have to exchange their energy with other
> electrons as they pass through, in a cascading fashion from one atom to the
> next-- which takes a finite time, from entry to exit. I'm not sure if this idea
> is in agreement with Feynman's explanation, though.

I don't think this is in agreement at all with Feynman's explanation. It 
can't explain why some materials have a refractive index less than 1 
(seen with xrays).

> By the way, another good source of info is Feynman's (small) book QED: THE
> STRANGE STORY OF LIGHT AND MATTER (about quantum electrodynamics.) It's
> surprisingly readable(!), without much math at all. Highly recommended.

Thanks for that, I shall look it up.

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