h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The blackbody curve comes from Planck's law http://en.wikipedia.org/wiki/Planck's_law , which is derived on the assumption that energy transitions are an integral multiple of some elementary unit energy; this is directly applicable to the case of phonon transitions, since a 'phonon transition' is a conversion of some integral multiple of phonons, which will all (at least, to the approximation of small intensities) have equal energy.

And electronic transitions aren't?

"Incandescence occurs when electrical resistive heating creates thermally excited atoms. Some of the thermal kinetic energy is transferred to electronic excitations within the solid. The excited states are relieved by pho-tonic emission. When enough of the radiation emitted is in the visible spectrum so that we can see an object by its own visible light, we say it is incandescing. In a solid, there is a near-continuum of electron energy levels, resulting in a continuous non-discrete spec-trum of radiation."

http://64.233.179.104/scholar?hl=en&lr=&q=cache:V1ifiaYLCvYJ:physicsed.buffalostate.edu/pubs/TPT/TPTDec99Filament.pdf+blackbody+filament

"And electronic transitions aren't?"

I don't quite understand what you are denying here.

(i) Electronic transitions aren't really multiples of any fixed single unit. Look at any electronic level diagram - the lines aren't going to be equally spaced. For *vibrational* energy level diagrams, the lines should be equally spaced (at least, those lines corresponding to the same mode), since the energy of a quantised mechanical oscillator goes as E = (0.5 + n) h w (h = Planck's constant, w = frequency, n = phonon population number).

Electronic transitions can be multiples of different 'level spacings', but these are, in general, not equal.

(ii) Before you were talking about a *continuum* of electronic states. By definition, a "continuum" is *continuous*. It doesn't make sense to say that the continuum is composed of a number of discrete, equally spaced energy levels (unless you're describe a spacing of zero).

Assuming the electronic states are continuous, the incandescence processes described in your linked article are using electronic states to mediate radiative transitions across phonon levels. Which is fair enough. I'll conceed that. If you are saying, in general, radiative transitions are forbidden between phonon levels, that's an interesting point. However, I'm not convinced that electronic transitions are *actually* required; for example, in the Raman effect, phonon generation occurs after an electronic transition... but if the Raman scattering is 'non-resonant', then that transition is virtual, only.

PS. I also said "the test for incandescence is usually..."

What I probably meant to say was "the test for incandescence is, within the literature that I commonly have contact with, usually..."

My experience is that terms like this ('incandescence') are used with a fair degree of flexibility and are usually defined only as rigidly as you want to use them in context.

If incandescence describes any light-emission-through-heat, then, yes, there will be, in some (or, the majority) of cases, emission following a line-like spectrum. However, I wanted to address black-body radiation, which I assume, is more properly a subset of all possible incandescence processes.

I think the quoted article says it all.

Yes there are radiative transitions between phonon states, however, the energy associated with these is not in the visible.

Phonon and electronic states are described by the corresponding "wavevector" of the quasi-particles (phonon or electron). The value of the wavevector for both changes continously from one extreme of the brillouin zone to the other. However, for a fixed value of k, there are quantized transitions.

Blackbody radiation occurs via both electronic and phonon transitions, however it is the electronic transitions that produce visible light. Perhaps if you don't beleive me should email the physics professors who wrote the above article and ask them?

whoa

hi
this is a very interesting topic... i know a little science but not enuf to understand the latest developments in this discussion...

when you guys finally figure it all out it would be so brilliant to MAKE AN ENTRY ON THIS SUBJECT including transparency, transluscence, opaqueness, iridescence, phosphorescence and incandescence ...and any other phenomenon i missed. say... one paragraph per phenomenon? with links for folks who want to really get into it... to entries (university piece) or outside websites?

and in language we non-physisists can understand... i could help with that part

physicists !

"Perhaps if you don't beleive me should email the physics professors who wrote the above article and ask them?"

I'm not sure I understand what part you think I don't understand.

""radiative transitions between phonon states ... is not in the visible.""

I would never deny that, if the transitions are restricted to small numbers of phonons. I said (very much) earlier, explicitly, I would expect single phonon energies to be on the order of a hundredth that of visible light.

"However, for a fixed value of k, there are quantized transitions."

I know and do not deny that. My point was whether the transitions occur across levels of *equal* spacing.

"however it is the electronic transitions that produce visible light"

Yes, it clearly states that in the article. I said in my recent post "maybe electronic transitions are needed to mediate a radiative phonon transition". That's what the article clearly implies. Unless you are suggesting that the article says [Everything in black body up to visible] -- purely phononic; [everything in black body within visible] -- purely electronic; [everything in black body above visible] -- (something else)... To be a decent black body spectrum, the power from the phononic and electronic parts must match, at the boundaries, pretty exactly. That surely can't realistically happen unless they are linked, in someway?

"I'm not sure I understand what part you think I don't understand."

Umm, maybe you should re-read what I wrote.