Re: Light slows down in glass?

["Bob Sciamanda" <trebor@velocity.net>]

Let me try another attack at this.  Start with a fixed point charge and its
electrostatic field.  Now introduce a conductor into this space;  there will
be a transient period of time during which the mobile charges  in the
conductor will be moved by the original field.  After the transient period,
the original field of the point charge still exists everywhere, just as
before, but there are now also the fields of the re-arranged mobile charges
(which will exactly cancel the original field inside the conductor).  The
net field everywhere is the superposition of all of these fields.

Now repeat the scenario, except that the original source charge is
oscillating and generating a wave into all of space.  Introducing matter
into this ALREADY EXISTING field will produce secondary radiation sources
out of each mobile charge IMMEDIATELY.  The original field still exists
everywhere, and there will be a transient period during which the secondary
and original waves superimpose into a steady state wave pattern whose PHASE
VELOCITY is c/n (given a transparent material).  The "extinction theorem" of
Omnes (1915) examined this model in detail (Cf Born & Wolf's Optics book).
Much like the phase velocity in a waveguide, this phase velocity can even be
greater than c!  It is only the "velocity" of a geometric pattern.
-Bob

Bob Sciamanda   sciamanda@edinboro.edu
Dept of Physics   trebor@velocity.net
Edinboro Univ of PA  http://www.edinboro.edu/~sciamanda/home.html
Edinboro, PA   (814)838-7185


-----Original Message-----
From: LUDWIK KOWALSKI <KOWALSKIL@alpha.montclair.edu>
To: phys-L@atlantis.uwf.edu <phys-L@atlantis.uwf.edu>
Date: Friday, February 06, 1998 2:17 PM
Subject: Re: Light slows down in glass?


> As I wrote several days ago, I am no longer satisfied with the "absorbed,
> reemitted, absorbed again" explanations. Any comments on what is below?
>
> > Date: Fri, 06 Feb 1998 10:48:05 -0500
> > From: "Richard W. Tarara" <rtarara@SAINTMARYS.EDU>
> > Subject: Re: Light slows down in glass?
> >
> > I would suggest that it is important to stress that electromagnetic
> > radiation always propogates at the same speed (speed of light) but that
> > when light enters a transparent medium it is absorbed, reemitted,
> > abosorbed again, reemitted again etc. and that this process makes
> > it _seem_ that the light has moved slower in 'passing through' the glass.
> ******************************************************
>
> > > Date: Tue, 27 Jan 1998 11:42:31 -0400 (EDT)
> > > From: LUDWIK KOWALSKI <KOWALSKIL@alpha.montclair.edu>
> > > Subject: Re: reflections and refractions
> > >
> > > .... It is an experimental fact that the speed of light in glass is
> > > about 2/3 of its speed in a vacuum. The Huygens principle ("it takes
> > > time for light to be absorbed and reemitted by electrons") was used
> > > to explain this behaviour. The value of v approaches c when the air
> > > pressure is decreased.
> > >
> > > I have a problem with solids and liquids. Let me introduce it in a
> > > very simple context. Light travels through a fiber cable whose diameter
> > > does not exceed one wavelength. (A technical term "monomode" is often
> > > used to describe this kind of an optical wave guide). The cable is one
> > > meter long. Suppose it is made from pure SiO2. The size of each
> > > molecule (3 nuclei and 44 electrons) is about 3*10^-10 meters and
> > > molecules are closely packed. We can thus say that the cable is
> > > composed of 3*10^9 "monomolecular layers".
> > >
> > > At the speed od 2*10^8 m/s the time to pass through the cable is 5 ns,
> > > or 2*10^-18 seconds per layer. Is this enough to abrsorb and reemit
> > > a wave? Keep in mind that the wave frequency is of the order of
> > > 5*10^14 Hz (T=2*10^-15 s). How can a wave be absorbed and reemitted
> > > during 1/1000 of its period?
> > >
> > > Also keep in mind that the phase shift introduced by a plate of glass
> > > of thickness lambda/4 (500 layers) is EXACTLY equal to 90 degrees. Does
> > > it mean that 5 layers of glass will introduce a 0.9 degree shift
> > > (without a wide spread of values around 0.9 degrees and without a large
> > > not-shifted component)? And if my recollection is correct, 5 monoatomic
> > > layers of a metal (evaporated on glass) are already very opaque.
> > > Something does happen to light in a layer whose thickness is very small
> > > in comparison with the lambda.
> > >
> > > Why is the speed of light in water (three nuclei and 18 electrons per
> > > molecule) higher than in SiO2? Why is speed of light in diamond (one
> > > nucleus and 6 electrons per "molecule") lower than in SiO2? ....