🚀 go-pugleaf

Johan Wevers (johan@blade.stack.urc.tue.nl) wrote:
: davids@spira.bri.hp.com (david sixsmith (contractor)) writes:
: 
: >1. What is (are) the inherent qualitie(s) of a substance, such as glass,
: >perspex, diamond, & water, which make it transparent ?
: 
: 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)
: 
: here, k=\alpha=\sqrt{\mu\omega/2\rho}. So, if \alpha is small, the wave
: can penetrate deeply wthout much absorption. This happens if \rho is large
: (bad conductor, like glass and perspex, and/or \mu is small (usually approx 
: \mu_r:=4\pi 10^{-7} H/m for this kind of materials).
: 
: >2. What is the mechanism which makes the speed of light lower when travelling
: >in such a medium as opposed to traveling in a vacuum ?
: 
: The interaction between the photons and the atoms in the medium.
: 

Thanks for your response.

I also had some response on e-mail. In fact (some what worryingly) with the
above equations there seem to be 2 rather different ideas (which may be
related);

	1. The equations above.
	2. The idea that transparent materials have gaps in their absorbtion
	   spectra which makes them unable to absorb photons in particular
	   frequency bands.

The second idea I find more convincing. Firstly because it gives me some
feeling for the mechanism involved (rather the just plugging numbers into an
equation), but also because (it seems to me) it answers the two rather puzzling
features of transparency - that materials seems to be either very transparent
or very opaque - and secondly that so few materials are transparent.

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

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

Incidentally - is transparency a proof of the relative smallness of the
nucleous ? Presumably photons will react in a head on collision with a
nucleous, and so if the nucleous was large compared to the atom, transparency
would never occur ? (Is this analysis correct ?)


On the second question answers seem to be shorter, but still varied. Anyone
prepared to be a bit more verbose than the above ? 

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| Dave Sixsmith                     |  Disclaim, disclaim, disclaim, etc.    |
| Bristol, England                  |                                        |
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