From: Richard.Mathews@West.Sun.COM (Richard M. Mathews)
Newsgroups: sci.physics
Subject: Re: Questions of transparency
Date: 15 Oct 1992 02:11:06 GMT
Organization: Sunsoft Inc., Los Angeles, CA.
NNTP-Posting-Host: astro
Originator: richard@astro.West.Sun.COM

prim!dave@germany.eu.net (Dave Griffiths) writes:

>Sorry, I'll rephrase the question. If one single photon enters the glass
>block, is it absorbed and retransmitted en-route, or does it pass straight
>through unimpeded?

The actual situation, as in all cases, is that what happens is the
superposition of all Feynman diagrams which fit the situation.  There
is exactly one diagram in which the photon goes through as the "same
photon":
		~~~~~~~~~~~~~~~~~~~~~
			time->
There are an infinite number of diagrams where in some sense (identical
particle behavior notwithstanding) it is a different photon.  These
include things such as
			        /
			       /
			      /
		~~~~~~~-------~~~~~~~
		      /
		     /
		    /
Here the photon scatters off of an electron.  In the sense that the two
photon trails are not connected, I will call these different photons
(but be warned that if you really insist on doing so, you are sure to
get into problems eventually).

What you see come out is the superposition of all of these diagrams.
The wave which survives the interference is dominated by those diagrams
which involve quite a bit of scattering.  Now, you can't exactly say
when and where it scattered.  But you know it must have scattered.

>If the photon _does_ fly straight through, why does it slow down? The gaps
>between atoms are a vacuum and it should whizz through at the speed of light!

It slows down because you get completely destructive interference between
the original wave and all of the waves which include scattering but still
have the photon come out of the glass at the time corresponding to the
photon continuing at the vacuum speed.  On the other hand the waves
corresponding to scattering and then coming out a little later (along
with the one wave corresponding to not scattering but still slowing down
anyway) constructively interfere, and that is what you see.

>Finally, if the hand-wavy argument is that the energy of the photon is
>transferred into oscillating fields of charges in the glass block... how?
>If nobody had yet discovered transparency, could QM have predicted it,
>together with the index of refraction and speed of light given the
>molecular structure?

Absolutely QM can predict it.  If none of the electrons is in a position
to absorb that particular photon energy and hold onto it, then the photon
can't be absorbed -- just scattered.  For example, silicon is transparent
in IR because the bandgap energy corresponds to IR frequencies.  Visible
light is able to kick an electron across the bandgap, but IR isn't.  Only
when you sufficiently dope or bias the silicon are there enough electrons
above the bandgap to absorb any significant amount of IR (and at that
point the silicon is a conductor rather than semiconductor).  Glass is
an awful lot like silicon (in fact it is mostly silicon), but it has a
bandgap bigger than visible light energies (or if there are states in
that energy range, transitions to them are forbidden by something such
as conservation of angular momentum).  The existence of discrete energy
levels is QM is what makes transparency transparent.

     Richard M. Mathews			G eorge
					 D epression
Richard.Mathews@West.Sun.COM		  B ush casts a
					   G loom on us all