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Re: speed of light of different colours



On Wed, 9 May 2001, julie wrote:

why is the speed of light for red bigger that , say, blue, when both are
in an optcally denser medium? I know the refractive indices for both of
them are different, and wavelengths are also different, but what
actually causes the "difference" ?

Because of an ultraviolet resonance of the atoms of the material. It's
because glass (etc) is made from atoms rather than from perfectly uniform
"stuff." At certain frequencies the light waves will electrically pump
the charges in those atoms into resonance. Close to resonance, the speed
of light "goes crazy" and is much higher or lower than usual. The speed
of light at frequencies far from resonance is fairly constant with
frequency.

If I recall correctly, the graph of the speed of light near a resonance
looks like this (wave speed versus frequency)

|\
| \
| --__
____________ | --------__________________
----------__ |
-- |
<-- low freq \ | high freq -->
\|


( graph needs Courier font)

I think that for most lens materials the important resonance is in the far
ultraviolet, and the "tail" of the resonance curve intrudes down into the
visible spectrum. It makes blue light propagate slower than red. There
are also resonances down in the infrared (which can screw up fiber optic
long distance communication that uses IR lasers.)



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