20th September 2008 - 02:25 PM
When light enters perfect dielectric is only gets"slowed" down.
But if the dielectric is not perfect, then absorption occurs.
We can have different mechanisms that all have the same effect: subtracting energy from the light beam.
one absorption mechanism is the electronic conduction: if the dielectric has a small, finite, conductivity, then currents will be generated and energy dissipated by Jolule effect.
But what if, still in theory, the conductivity is zero?
Energy can still be absorbed by resonance (if the frequency is in a certain range).
How is that taken into account in Maxwell equations?
( I only always see the conductivity as responsable for absorption).
What are the Lorent-Lorentz, Drude models?
21st September 2008 - 08:09 PM
Well, if light is absorbed by transitions in molecular orbitals, you may represent it by a conductivity. Just remember that you need enough particles for such equations to make sense.
A conductivity isn't completely abstract here, as the absorbing electron changes from one orbital to another, and both have different forms with different electromagnetic properties, so you can say that the electron has moved in the electric field. In fact, light won't make an electron move from a spherical orbital to another spherical one, for instance.
Drude is a model of conduction and polarization current in a metal as a function of frequency. It assumes that electrons have a mass and a mean time-of-flight. Simple and efficient. Have a look at Wiki.