Re: Light, virtual processes don't explain refraction

[John Denker <jsd@MONMOUTH.COM>]

At 08:03 PM 10/21/99 -0700, Leigh Palmer wrote:

> the absorption doesn't have to be resonant absorption,

Right.

> Considering light as being absorbed and reemitted by electrons
> in a transparent medium entails the processes being virtual;

> and the
> time during which the electron is excited to a forbidden state
> is within the bounds of what is allowed by the uncertainty
> principle. That is, the amount by which the light is nonresonant
> times the duration of the absorption-reemission process is less
> than or of the order of Planck's constant.

That's an interesting guess, but it's not the right explanation for glass.

(There are questions in physics for which short-lived virtual states are
the answer, but refraction in glass isn't one of them.)

To get a handle on what is or isn't going on here, we must pin down the
notion of "duration" that figures so prominently in the suggestion.  There
are two main timescales in the problem:
 *) One timescale is the typical "ringdown" time of the atomic oscillators.
 This is roughly Q over omega.  If this timescale sets the called-for
"duration", then the suggested process only works if the incident light has
the same frequency as the atomic oscillator to within one part in Q.  Since
atomic oscillators have rather high Q (typically millions, or at least
thousands) and since light refracts even when it is thoroughly off
resonance (an octave or more), this explanation doesn't wash.
 *) The other timescale is roughly the inverse bandwidth of the incoming
light.  If the light is a very short pulse, its frequency distribution will
indeed have tails that overlap the resonant frequencies of the atomic
oscillators.  But this explanation predicts that refraction depends on
having short-pulse light, and totally fails to predict the refraction of
steady monochromatic light.

======================

So let's forget about short-lived virtual processes and stick to the
well-established classical description of off-resonance re-radiation.

See also Bob's note (Thu, 21 Oct 1999 23:42:09 -0400) about the Ewald-Oseen
extinction theorem.

______________________________________________________________
copyright (C) 1999 John S. Denker             jsd@monmouth.com