Re: Quantum Function of Mirrors

In article <t9lqg1l98u055cmt005feet4udnmu36cuc@xxxxxxx>,
softrat@xxxxxxxxx says...
> On Wed, 24 Aug 2005 14:40:29 +0000 (UTC), Gerry Quinn
> <gerryq@xxxxxxxxxxxxxxxxxxx> wrote:

> >Maybe that's correct in some sense, but intuitively it seems wrong.
> >Not all mirrors have substantially delocalised electrons - they may be
> >tightly bound within atoms. So you are treating electrons in different
> >atoms as the same electron, and yet the probability of them moving
> >between atoms is very low.

> Check your 'intuition'. If there are not any electrons 'free', as in
> metals, you will not get the reflectivity you need. (Why do you think
> that the finest mirrors are made of metal or metal films?)

Sure, metal mirrors are the best. But water, glass, and marble also
reflect light, and they don't have much in the way of free or (as we
were talking about) delocalised electrons. (I suppose it depends on
the material how big a 'molecule' the valence electrons associate with
in a solid.)

Electrons don't have to be free - what matters is that they will
rapidly return to their original energy state with re-emission of a
single photon. An outer electron bound in an atom or molecule can
surely do this without too much difficulty at visible light
frequencies. I assume that the electron does not get promoted out of
its original molecule, because if it did it would surely emit several
photons as it tried to find its way back, i.e. the light would be
absorbed and converted to heat.

I'd guess that this is certainly easier to achieve with electrons in a
partly-filled conduction band, hence the typical shininess of metals.
Also, the effective density of available and willing electrons is so
high that metals don't transmit light unless they are in very thin
layers such as gold foil.

However, materials without free electrons can still reflect light, so
they are not a necessary feature of mirrors.

Probably the above is all over-simplistic, and excitations other than
an electron going up to a high orbital and dropping down again are
typically involved. I suppose one could generalise, however, that to
make a mirror, you need either free electrons, or tightly-bound ones.
A semi-conductor would make a poor mirror, because there would be a
tendency for whatever excitations occur to send electrons into the
conduction band, where they would get lost and lose the energy as low-
grade photons.

- Gerry Quinn

.

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