"willi" <willi-1291@wanadoo.fr> wrote in message
news:BCF0D899.9C59%willi-1291@wanadoo.fr...
dans l'article E6xyc.86560$wd7.25103@front-1.news.blueyonder.co.uk, Keith
Stein à a écrit le 12/06/04 8:09 :
the speed of an
electromagnetic wave is determined by the medium it is travelling
through, while the speed of a photon is unaffected by the medium it
is travelling through
When you send a photon (i.e. For example desexciting an atom at a given
time) trough a medium of thickness L with refractive indice n(lambda),
exactly with a velocity group v<c
you think the photon will be detected at a time L/c and not L/v ?
!
The point is Willi that in the experiment proposed by Jim Greenfield
the speed comparison is between two MACROSCOPIC light beams,
not individual photons eh!. Now what i know is that macroscopic
light beams require a macroscopic medium, and the macroscopic
light beams travel relative to the macroscopic medium, as predicted
by Maxwell.
As to what individual photons do, i'm really not sure, but people who
do claim to know, like Mr. Austin, seem to think that they always at c eh!
Message 11 in thread
From: Jack Austin (zythos2@home.com)
Subject: Re: THE SPEED OF PHOTONS IN WATER ?
View this article only
Newsgroups: sci.physics
Date: 2001-04-30 03:51:08 PST
"keith stein" <> wrote in
<_NPG6.7611$Mz.1053149@news1.cableinet.net>:
Someone once told me that between water
molecules photons always travel at 'c(vacuum)',
It is my understanding that individual photons always travel at c.
However, if the light is only slowed down because
of temporary absorbtion by water molecules,
then i don't understand how light can travel in
a straight line underwater - unless the photons
are always reemmited in exactly the opposite
direction from which they were absorbed,
of course, but that do seem most unlikey to me.
If photon(s) are absorbed into a stationary state of an atomic or molecular
electron, the photon(s) are trapped until something imparts some additional
energy (e.g. another photon or a collision). It is possible that the
additional energy will come from a collision or a photon from the vacuum
field. In which case, the emitted photon can take off in any direction. If
the additional energy comes from a photon which is identical to the photon
which was absorbed (same direction, polarization and frequency),
conservation of energy says the emitted photon will take off in the same
direction, with the same polarization and with the same frequency as the
original photon.
In your case, it is more likely that the incident photons will not be
absorbed at all. Absorption and remission will only occur at frequencies in
the neighborhood of the natural mechanical resonances of the charged
particles (electrons, protons, etc.) comprising the water molecules. The
majority of photons in white light passing through water are not undergoing
absorption and reemission. Rather, the incident light is coupling with the
electrons in the water molecules and causing them to move in an oscillating
fashion. Whenever charged particles accelerate and decelerate, they give
off radiation. Since the acceleration and deceleration of the electrons
lags with respect to the incident field, the phase of the generated
radiation lags with respect to the incident radiation. The resultant sum of
incident and generated radiation, lags with respect to the incident
radiation. The phase lag accumulates as the radiation propagates through
the water, resulting in a propagation velocity which is less than c.
So what is the speed of photons under water eh?
The propagation velocity of individual photons = c
The propagation velocity of light = c / 1.33
keith stein
.
|
