From: pauld@exploratorium.edu
Date: Mon Apr 03 2000 - 22:42:04 PDT
Message-Id: <200004040542.WAA23553@isaac.exploratorium.edu> From: pauld@exploratorium.edu Subject: Re: pinhole Scattering Date: Tue, 4 Apr 2000 05:42:04 GMT
Hi Sidney
Light can change direction by being absorbed by the atom or molecule, causing a
change in the energy level of the electron, and then re-emitted. Such an
absorption and re-emission might take 10^-8 seconds.
Quasi-elastic scattering is the deflection of light by atoms or molecules where
the light exerts forces on the electrons in atoms or molecules causing them to
move. The light exerts sinusoidal forces so the motion of the electrons is
sinusoidal at the same frequency.Oscillating electrons re-radiate the light
energy. Since the electrons are bound to the atoms or mlecules they have a
resonant frequency, the closer the light frequency is to this resonance the
greater the motion of the electrons and the more energy is re-directed.
When you drive a resonant system you get a phase delay (or advance) between the
driving frequency and the re-emitted frequency. This delay is around 10^-15 s
much shorter than the absorption-re-emission time given above involving
absorption and re-emission with a change in energy level.
The phase advance is interesting in that it produces speed of light in a
material greater than the speed of light in a vacuum, in metals for instance.
This is the phase velocity of light and so causality is not in danger since
information can only travel at the group veocity, which is less than the speed
of light in a vacuum.
Paul Doherty
> On the subject of blue skies, I realize that I don't really understand what
> "scattering" is in the first place. I take it to mean the deflection of
> light rays by atoms or molecules, but I don't know what the mechanisms might
> be by which the rays are deflected. Is resonance between the light and the
> atoms or molecules always involved? Are there other means besides resonance
> by which molecules scatter light, and if so what might they be?
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