On 08/01/09 13:54, Darren New wrote:
> I think the way it works is this:
>
> X-rays actually are small enough to go between the atoms (altho they'll
> still interact with the electrons, which is why metal still stops them,
> having a "sea" of electrons on the surface).

	Here I'd rather view them as photons, to whom I do not want to attach a 
size. Perhaps the wavelength can be used to "fake" a size of the photon, 
though.

	Keep in mind that everything I say is a bit of guessing and not 
actually studied (well, not all of it at least).

> Visible light hits (most) atoms and gets absorbed, reflected, etc.

	Essentially, an electron in an atom will usually absorb a photon only 
if the photon has an energy such that there is an available energy level 
for the electron to go to. For insulators, that means it will (usually) 
have to cross the band gap. For metals, there is no (initial) band gap 
and so electrons of low frequency are readily absorbed. Perhaps that's 
why metals can easily get in the way of radio waves?

	However, I think there are higher bandgaps in metals, and so photons of 
a certain energy range will pass through.

	(Insulators also have higher bandgaps).

	There's no "true" reflection. Quoting Wikipedia:

"Light waves incident on a material induce small oscillations of 
polarisation in the individual atoms, causing each atom to radiate a 
weak secondary wave (in all directions like a dipole antenna). All of 
these waves add up to specular reflection..."

	For stuff like refraction, essentially, the material will get polarized 
by the fields of the photons trying to pass through it. This will result 
in internal electric fields that interact with that of the photon, and 
result in a reduced speed, change in direction, etc. Essentially, the 
relative permittivity that we used in physics courses is simply a model 
of the effect of polarization on the external waves.

	I find it disturbing that Wikipedia's article on refraction has no 
actual physical explanation, but I suspect I'm close. 	

> Radio waves are physically bigger than the atoms (and the whole house,
> for that matter) so they basically are ghosting along like a car through
> air.

	For me, I'd prefer to view it more in terms of photons interacting 
electrically with the solid (and/or taking into account the band 
structure) rather than size. Although as I said, perhaps one could 
produce an "effective" size which matches both what you say and the 
actual theory - although there may be materials that fail the "size" 
theory.

	When I studied solid state physics, we didn't cover interactions with 
light. And I never thought to see if there was a pattern between lattice 
constants (the spacing between atoms in the crystal), and their band 
energies. I actually expect that the relationship between the band 
energies/gaps and lattice constant is poor/nonexistent - which may 
weaken your approach of using size for intermediate frequencies.


-- 
Kotter: "Have you ever considered becoming a vet?"
Epstein: "Uh...Uh no. My brother Sanchez was in the army. Didn't like it 
a bit."

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