Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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? ....