🚀 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?

In a solid the electrons on the outside of each atom (molecule) form
bands.  It is the energy difference between these bands that determines
the absorbtion properties.  Because the bands are composed of many atoms
the energy levels are quite broad.  Each atom has to be in its own state
but there are so many states that the probability is very high that
a photon in the right range will be absorbed.  If the band gap is large
only high energy photons will be absorbed, the lower energy will pass
right thru because the photons can't push any electrons out of the lower
bands.
 
> 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?

Absorbtion spectra are not flat.  There is a huge dip in the absorbtion
of light by water in what we call the "visible" range.  An interesting
connection to evolution.  The dielectric constant (isn't at all constant)
is a function of electron density, structure and wavelength.  Group
velocity is related to dielectric constant, so it is hard to predict.
Probably better to say "this is what we measure".

> 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?

It depends :-)  If the light is absorbed, then the emitted frequencies won't
be the same as the absorbed ones because of particle interactions.  Heat
in other words.  Microwaves heat water, not pasta or ceramic.  Microwave
ovens are designed with water in mind.  The resonant frequency is a
vibrational mode (several are closely spaced).  The vibrating water
molecule hits a different kind of molecule (protien say) and transfers
some of its energy.  Now, it can't emit the original energy because
it no longer has it, and the stuff around it is heating up too.  The 
vibrational mode will be different too, eventually the water molecule
will transfer all its vibrational energy to translational (motion) and
just be hot.

Try heating ice in a microwave.  It doesn't work very well until a little
bit of water is present.  Then it goes fast.  The ice can't absorb the
microwaves, it just scatters them.  The water can absorb them and can
then couple the energy to the bound molecules in the ice.  Try heating
dry pasta.  It won't get hot at all, there is no water and no band
gap close to the microwave frequency.

Hope that helps,

Patience, persistence, truth,
Dr. mike

> 
> Thanks very much,
> 
> Kevin Luttrell
> Polytechnic High School