Re: Light, resonance, index of refraction,

[John Denker <jsd@MONMOUTH.COM>]

At 09:11 PM 10/21/99 -0500, Cliff Parker wrote:
> What is going on as light passes through various mediums.

> I understand that at
> least part of the story can be explained as a process of
> light energy being absorbed and reemitted by electrons
> within the medium.

Yes.

> This absorbance and reemission takes
> some time and thus light is slowed down.

Roughly speaking, yes.

> The question I am
> pondering is "How does a substance like glass transmit all
> colors?"  Is it that the structure of glass offers so many
> different arrangements of atoms that virtually all energies
> represented by visible light can be absorbed by some
> electron somewhere within the substance?  What happens to
> light whose energy is not correct for absorption within the
> medium?  Why wouldn't some wavelengths of light simply pass
> through the medium untouched at 3.0 x 10^8 m/s?

This question implicitly assumes that light is either absorbed on-resonance
by some narrowly-tuned high-Q oscillator, or not absorbed at all.  Reality
is much different -- essentially *all* the optical waves are off resonance.

So let's start by considering off-resonance absorption in a model system,
namely an ordinary mass on a spring.  If you walk up to a mass on a spring
and steadily push on it, the mass moves!  All you have to do is compress
the spring.  If you push on it sinusoidally, the mass moves sinusoidally at
that frequency.  If the mass also carries a charge, it will radiate at the
frequency of your push.

(Of course you could get more motion per unit force by pushing on
resonance, but the point is that you get definitely nonzero motion off
resonance.)

In good glass, virtually all of the re-radiators have resonances *above*
optical frequencies.  Optical light hits them below their resonant
frequencies.  Ultraviolet light does hit them on resonance, and is so
strongly absorbed that glass is opaque at such frequencies.

We get two bits of physics for the price of one:  this explains why you can
make a prism.  As the light frequency goes up, closer to resonance, it
interacts more strongly with the glass.  I'll bet you've never seen a type
of glass which (when made into a prism in the usual way) would bend the
blue rays less than the red rays.  In reality the blue bends more than the red.

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