🚀 go-pugleaf

Kevin Luttrell wrote:

> 1) Violet light is slowed more than red light as it passes through glass
> because violet "interacts" more with the electrons therein. As I
> understand it, this is because most glass absorbs UV frequencies which are
> closer to violet than to red. If I treat electrons as harmonic
> oscillators, I can imagine this. But from the quantum mechanical
> perspective, why would certain molecules have electrons that absorb UV,
> but not IR? I know it has to do with their arrangement (orbitals, and
> bonding, and so on), but I'd like to understand this better from a quantum
> mechanical perspective. Is it the electron's tendancy to leap from energy
> level x - to - energy level y that determines its resonant frequency?

You really can't think of individual atomic levels or binding when
you'rethinking about energy levels in a solid.  And different solids have
different dominant
mechanisms for absorption.  The UV edge in glasses and semiconductors and
dielectrics
is due to the forbidden energy gap, which you can illustrate to yourself by
solving the
Kronig-Penny potential.  There are also a number of things that tend to
broaden this (aside
from the normal thermal broadening), but these are too numerous to get into
here.  Energy
gaps in periodic (or quasi-periodic or even amorphous) solids are one of the
fundamental
victories of solid state physics - this is what dictates that many of (the
electrons in) these
solids cannot absorb radiation below a certain energy.  Electrons do not
simply have a
"tendency" to change energy levels, they are constrained to change levels at a
certain
minimum of energy.  This is not a resonance phenomenon, but a property of the
solid.
It is analogous to the levels of a hydrogen atom, save for the fact that the
conduction and
valence bands in a solid can be "quasi-continuum" states, much broader energy
spectrum and
much less localized than a hydrogen atom.

That said, there are other mechanisms for absorption, including many which
happen in the IR.
Light can excite phonon modes (quantized ionic vibrations) in the solid - this
is particularly
significant in glasses; silica, for example, cuts off in the IR due to
multiphonon absorption.

> 2)  And water absorbs microwave frequencies, so why in the world does it
> also slow violet more than red? Shouldn't it be better at slowing the red
> because red is closer to the frequencies it absorbs?

In water, as in most liquids, there are a gazillion absorption mechanisms.
The electronicstates tend to absorb in the UV, but in the IR you have
molecular vibrational and rotational
states as well, along with phonons.  The absorption spectrum is nowhere near
as simple as
many solids.

> 3) Then there's scattering. Are the frequencies that get emitted the same
> ones that were absorbed? And besides, I thought that if you hit a molecule
> with its own resonant frequencies that it would absorb them and heat up,
> not re-emit them. So why do microwaves heat up water molecules, and not
> get scattered?

Scattering is different from absorption.  When you "hit a molecule with its
ownresonant frequencies" you are (usually) exciting electronic states, which
then
re-radiate the same frequency as the electron decays to the ground state.
Microwaves heat water molecules because the 2.064 or 2.45 GHz bands are
absorbed by water and then dissipated as heat (they usually excite rotational
modes which cause the molecules to scatter off each other).

For elastic scattering, f_in = f_out.  For inelastic, it's not true.  There
are also
many different types of scattering, from Rayleigh (the blue sky) to Raman
(used
by spectroscopists & now some interesting device applications).  Raman
scattering
is a little bit like a combination of ordinary scattering and phonon
absorption.
It's important to keep the different mechanisms clear, however.

I hope this helps.

David