🚀 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....

This is a common problem; the resolution is that there are really
three or four (at least!) separate effects going on at once, which
is why the absorbtion response is hard to understand.

First, there is the quantum-mechanical absorption-resonance.  When the
photons (light) have the same energy as an atomic energy level (so that
an electronic bound state can be promoted to a higher-energy bound
state) then the atoms will absorb the light rapidly.  Note that re-emission
will also occur, because it is a higher energy state, and thus not favored;
this resonant and re-emission process will reach an equilibrium, so that
at first primary absorbtion occurs, but after long times there is a healthy
mix of both.  This is how lasers function.  The quantum-absorbtion energy of
most atoms is in the visible to UV.

Second, there is the mechanical oscillation effect.  That is, atoms in a
lattice (harmonic oscillators, solids, etc.) will move around in response
to an applied electric field (such as light).  There will be some resonant
frequency for this motion as well (the plasma frequency) which changes
dramatically with material density.  It is this effect which is responsible
for the variation of the speed of light in materials (the lines of atoms
react to the applied field but also emit new field; they can end up
"advancing" the field by their emissions).

>2)  And water absorbs microwave frequencies, ...

This is yet a third effect.  Many molecules, like water, also have a
rotational moment, and a set of quantum rotational energy states.  These
are much lower energy-difference states, so they are resonant in the
Infrared.  A microwave functions by causing the water to wiggle, thus
heating up.  (as above, this will be an equilibrium process of absorbing
and emitting, but at each step of the process the energy might turn into
"heat" = random motion, instead of light).

There are of course even more effects (spin resonances in the electrons
and nucleus, etc.) giving the absorbtion spectrums of molecules a great
breadth of character (many chemists make their living trying to interpret
all the strange peaks in the infrared; rotational moments of molecules are
notoriously difficult to track down..)

>3) Then there's scattering. Are the frequencies that get emitted the same
>ones that were absorbed? 

This is yet another complicated process, because there are many different
types of scattering.  In stimulated-emission processes, as I mentioned
above for lasers, light hitting atoms at their resonant frequency will be
re-emitted at their resonant frequency (this is due in part to the bosonic
nature of light).  There are also "brehmstrahlung" and "compton" scattering
processes by which a particle slows down slightly and the atom or electron
speeds up slightly; this is a non-resonant process so that a continuum of
frequencies can be produced (but usually only high-energies take part in
these processes).  An even the mechanical response of the electrons is
a sort of continuous scattering and re-emission process..

---------------------------------------
Charles Bloom    cbloom@caltech.edu
http://www.cco.caltech.edu/~bloom/