🚀 go-pugleaf

davids@spira.bri.hp.com (david sixsmith (contractor)) writes:

>: The differential equation for an EM wave traveling in matter is given by
>: (For E only, a similiar equation holds for B):
>: 
>: \nabla^2 E = \ddot{E}/c^2 + \mu\dot{E}/\rho.
>: 
>: were \mu is the permeability and \rho the resistance per unit of length.
>: This equation has plane wave solutions:
>: 
>: E=E_0 exp(-\alpha x) cos(kx-\omega t)
>
>	2. The idea that transparent materials have gaps in their absorbtion
>	   spectra which makes them unable to absorb photons in particular
>	   frequency bands.

This is right, of course. The mechanism of the damping mechanism I described
above is that a varying E field generates a current. These equations assume
that this current can be described classically.

If you look at it more close, you see that the QM description of material
gives the corrections which others told you: not all currents behave like
they were classically.

>The proof of the pudding would be to predict transparency in a material. Does
>anyone know if this has been done ?

Yes, you must calculate which frequencies give which resonance. The better
the resonance, the better the absorption. These calculations can be done
semi-classically with (as far as I know) qualitative good predictions.

>It seems remarkable that we have here a day-to-day phenomenon which needs
>some understanding of QM to give a explanation. Does anyone know what the
>classical explanation was;

The generation of electrical currents. The explaination I wrote was classically.
Without QM, the absorption bands are not understandable.

>after all questions of lenses etc were well
>understood by Newton - did noone have any idea why glass was transparent.

He did have some difficulties with collor correction: Newton tought that it
was impossible to give the focus point of 2 collors the same position.

>Incidentally - is transparency a proof of the relative smallness of the
>nucleous ?

No, interaction cross sections have nothing to do with the size of the
pasticles (they are assumed to be point-like anyway). Cross sections can
be calculated with higher-order quantum electrodynamical corrections.
-- 
********************************************************** 
* J.C.A. Wevers                *   The only nature of    * 
* johan@blade.stack.urc.tue.nl *   reality is physics.   * 
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