Re: How does light travel through a vacuum?


srp wrote:
> shevek4@xxxxxxxxx a écrit :
> > Timo Nieminen wrote:
> >
> >>On Fri, 29 Dec 2005, shevek4@xxxxxxxxx wrote:
> >>
> >>>Gregory L. Hansen wrote:
> >>>
> >>>>>An atom emits a photon.. by dropping from one discrete energy state to
> >>>>>another. That contradicts your statement that photons are not about
> >>>>>discrete energy states.
> >>>>
> >>>>There are photons associated with that, sure. But they just conserve
> >>>>energy and quantum numbers. Dropping from one discrete energy state to
> >>>>another is more about discrete energy states, the way I see it. Photons
> >>>>come along for the ride.
> >>>
> >>>They are absorbed or emitted for the ride - a quick ride giving them a
> >>>precise quantum of energy in the rest frame of the system. This
> >>>(photon) radiation is of a very different character than the other kind
> >>>of light we were discussing - slow or continuum emission such as
> >>>cyclotron radiation or radio waves from an antenna.
> >>
> >>[cut]
> >>
> >>>>They're also produced in synchrotrons. Probably the most important use of
> >>>>synchrotrons is to generate x-rays.
> >>>
> >>>So please tell me how this kind of light is quantized! The light is
> >>>generated by the acceleration of free electrons by magnetic fields.
> >>>The energy of the free electron is not quantized, nor is the magnetic
> >>>field strength (those two parameters can easily be adjusted as
> >>>experimental parameters). How could the emitted light be quantized?
> >>
> >>How indeed? But were not the first definitive photon experiments
> >>(Millikan's photoelectric effect experiments) absorption of photons by
> >>conduction electrons in metals, with unquantised energies both before and
> >>after absorption?
> >>
> >>The spectra due to free-free and bound-free atomic transitions matches
> >>theory if you assume quantisation, E=hf. How are free-free atomic
> >>transitions fundamentally different from synchrotron-accelerated
> >>electrons?
> >>
> >
> >
> > E=hf does not necessarily imply quantization. If the frequency can
> > vary continuously, then so can the energy, and you can have any energy
> > photon you'd like. The formula applies to emission and absorption of
> > energy. "Free-free" atomic transitions are perhaps somewhat misnamed -
> > if they are atomic transitions then they are governed by the central
> > potential of a nucleus, and can act to quantize the energy - only
> > discrete energy eigenvalues are solutions. Just because the captured
> > electron can quickly be transferred to neighboring atoms due to its
> > valence nature doesn't mean that the electron is "free" - it is still
> > bound by the atoms of the crystal.
> >
> > A key is your use of the word "absorption" - something that is
> > absorbed isn't free. Synchrotron accelerated electrons see no central
> > potential, atomic transition electrons do see a central potential.
>
> Actually they do, kind of. They are forced to move in locally
> uniform orthogonal electric and magnetic fields around a very
> precise approximately circular path.
>
> The precision and intensity of the fields allow setting very
> precise energies to be released as synchrotron photons, that
> in my view, just about amount to forced quantization.
>
> I refer you again to that excellent textbook
> "Principles of Charged Particle Acceleration" by S. Humphries Jr.
>

Thanks for your reply.

You may want to note that a Hydrogen atom, unlike a synchrotron, cannot
be tuned to emit light of any frequency. Of course this is true for
all atomic emitters and this solved the "ultaviolet catastrophe" of
blackbody emission.




>
> >>--
> >>Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
> >>E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
> >>Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
> >
> >
>
> --
> André Michaud
> Service de Recherche Pédagogique http://pages.globetrotter.net/srp/

.

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