On 30 Sep 1999 10:29:58 -0400, Nieminen Juha <war### [at] cctutfi> wrote:
>Ron Parker <par### [at] fwicom> wrote:
>: The explanation I always got is that the photon is being absorbed and
>: re-emitted each time it hits a water molecule, but it is traveling at
>: c in the interstices between molecules.
>
>  The speed of the photon must slow down at least somewhere so that
>the average speed could be less than c.
>  If the photon is absorbed by a molecule, why does the photon leave the
>molecule in the exact same direction as it entered it?

Okay, new data.  This is from Feynman's _Lectures on Physics_, which 
everyone should own.  He does a non-differential-equation derivation
in chapter 31 of volume 1 for gases (because if the atoms are too 
close together you get interactions that are too hard to account for
without differential equations) which he then summarises in chapter
32 of volume 2 thusly:

  The electric field of the light wave polarizes the molecules
  of the gas, producing oscillating dipole moments.  The acceleration
  of the oscillating charges radiates new waves of the field.  This
  new field, interfering with the old field, produces a changed field
  which is equivalent to a phase shift of the original wave.  Because
  this phase shift is proportional to the thickness of the material,
  the effect is equivalent to having a different phase velocity in
  the material.

This is the wave-based version rather than the particle-based version,
but it makes more sense, I think.

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