Re: A few basic physics Qs
- From: Andy Resnick <andy.resnick@xxxxxxxxxxx>
- Date: Tue, 17 Jan 2006 08:54:07 -0500
Stephen wrote:
<snip>I couldn't spot any answers to these questions in the FAQ, so please accept my apologies if any of them are redundant.
3) Are there any known quantisation restrictions on the energy of a
photon? E.g. Say "valid" photon energies are 3.001 eV and 3.003 eV, but
no 3.002 eV photons can exist?
Yes and no. When a photon is emitted due to an electron transiting (transitioning?) between two bound states, the photon energy is quantized and has certain predictable values. For a photon emitted by an electron transiting between a bound and unbound state (or vice versa, or between two unbound states), there are no quantization restrictions on the available energy values. More generally, allowed energy states in bound systems are quantized, but free particles have a continuum of available states.
While I'm at it, do emission spectral
bands have an actual width? Or are they depicted as thicker sections just to indicate luminosity, due to certain transitions being more likely?
Because energy and time are conjugate variables (dE*dt=hbar), the spectral width of emission lines is not zero, and depends on the "amount of time" an electron stays in an excited state. If an excited state is very unstable (like pico- or femto-second), the emitted spectral width is large. If the excited state is long-lived (like nanoseconds), the spectral width is very narrow. Note, this is just a discussion about photon emission and spectral width: scattering is different, and spectral width there arises from the continuum of possibilities for partitioning the energy between a photon and a, for example, phonon.
4) Does the "amplitude" of a photon (or the amplitude of the "de Broglie wave" of a moving particle) have any meaning?
Without getting into a philosophical discussion about "meaning" or whatnot, the amplitude is not generally measurable. For photons, I believe the amplitude is measurable for long-wavelength cases, like radio and the 60Hz electrical power and so on, but not for visible photons. Sometimes it's referred to as "coherent" or "incoherent" detection, and digging into this will get you into some nice expositions about density matrices.
<snip>
Thanks!
No problem.
-- Andrew Resnick, Ph.D. Department of Physiology and Biophysics Case Western Reserve University .
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