Re: light is slowing down

[LUDWIK KOWALSKI <KOWALSKIL@alpha.montclair.edu>]

Prompted by a private message (see below) let me explain why I think that 
Huygens principle is an objectionable argument for v<c in glass and water. 
(The author calls it "a smokescreen for the scatterning process". I would
not go that far; a military smokescreen is usually a deliberate attempt to 
cover the truth. An honest misconception is probably a better description.)

The introductory model of "absorbing and reemitting " of waves is a setup 
of two tuning fork boxes, one ringing and another initially at rest. After 
many periods the second fork box starts ringing and emitting waves. I used
to say that something similar happens to light; each molecule on the path of 
a wave is like a little tuning fork, except that forces are electric rather 
than mechanical. The secondary wave fronts we draw, for example to derive 
Snell's law, represent waves from secondary sources. But I am no longer 
comfortable with the "absorprion and reemission" explanation. Ligh travels
too fast to make it acceptable. Can a replacement suitable for an 
introductory physics course be found for explaining v<c?

                                                  Ludwik Kowalski
P.S.
Why does the "out of phase" (see below) must mean v<c? Note that the speed 
of light in air decreases gradually with pressure. I used to say that the 
speed of light decreases when the number of "absorption and reemission" 
steps, per unit length, is growing. How can v<c in air be explained 
correctly at the elementary level? And why only some dielectric materials 
are transparent?

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> I'm entering this conversation late... so perhaps this has already been 
> mentioned and I missed it. Have you looked at Feynman's approach to the 
> index of refraction? It seems to me that the 'absorption and reemmsion 
> etc' are really just a smokescreen for the scatterning process. An 
> incoming wave induces a time dependent polarization of the atomic charge 
> distribution that can then produce it's own radiation... out of phase 
> with the original wave of course... I think it's the influence of this 
> additional wave that produces the change in speed of the origianal 
> wave... at least that's my understanding.
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