Re: Does "c" loose some velocity after leaving a dense medium back into space?
From: Harry (harald.vanlintel_at_epfl.ch)
Date: 03/23/05
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Date: Wed, 23 Mar 2005 12:36:47 +0100
"Tom Roberts" <tjroberts@lucent.com> wrote in message
news:d1nk0n$96k@netnews.proxy.lucent.com...
> richard miller wrote:
> > "Bill Hobba" <bhobba@rubbish.net.au> wrote in message
> > news:VvG%d.6356$C7.5337@news-server.bigpond.net.au...
> >>Each photon when created travels at the velocity of light until it
> >> interacts
> >>with something. In a medium it is the interaction that causes it to
slow
> >>down. The photon undergoes absorbtion and reemission in the medium
during
> >>which it may loose energy.
>
> That's one way of looking at it, but not a verry good one -- that
> essentially enshrines the perturbation APPROXIMATION to QED in your
> description.
>
> It is better, and more faithful to QED itself (rather than an
> approximation), to say that in a medium the interference between a
> traveling light ray and the charges in the atoms cause the light ray to
> be measured as traveling with speed c/n relative to the rest frame of
> the medium; here n is called the index of refraction and characterizes
> the density and masses of the charges in the atoms of the medium.
Indeed, as already Michelson suggested, a better way to look at it is that a
medium causes a propagation time delay without slowing down the light when
it travels in the space between atoms. And energy loss corresponds mostly to
absorption, although in principle there should be an extremely small
redshift with every interaction. Please allow the following precision:
According to standard theory, light propagates as measured in the rest frame
of the medium at c except when interacting with the medium during which time
the speed is measured as c/n in that same frame. In a frame that is moving
relative to the medium, the respective measured speeds are c and approx. c/n
+ v(1- 1/n^2).
> Outside the medium, of course, the density of charges is VASTLY less, so
> there is correspondingly less interference affecting the phase of the
> light ray, so it is measured to travel with speed c.
>
> Note I am discussing the theoretical predictions of QED. Real
> experiments, however, agree with these predictions to within
> experimental resolutions, which are typically a few parts per billion
> for such measurements.
>
SNIP
Harald
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