Incandescent light clarification

  1. Chi Meson

    Chi Meson 1,772
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    I understand the basics of blackbody radiation and incandescence in general: as the temperature of objects increases, atomic vibration increases, electrons are knocked-up, then de-exite releasing photons, etc etc.

    My question is, why is the blackbody curve completely free of signature spectral lines caused by the elemental make-up of the radiator? Why, for example, does the radiation curve not look like an x-ray emission, with both the bremstralung curve, plus characteristic k-lines?
     
  2. jcsd
  3. Firstly, a blackbody curve is by definition free of material-specific lines. In practice it's probably difficult to produce perfect black body radiation, but (noting it's also sometimes called "cavity radiation") one way is to produce a cavity inside a material (with a large internal surface and a small exterior window), so that the radiation has time to reach an equilibrium before it escapes. At first a material might emit a few wavelengths more strongly, but it will also absorb them more strongly, whereas other wavelengths (having a weaker interaction with the material) are reflected better..

    As for incandescent lights, I'm not sure to what extent they do additionally produce characteristic lines in their spectra?
     
  4. ZapperZ

    ZapperZ 30,437
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    Incandescent lights produce light via the vibrational spectra, not via atomic transitions. It is just heat. Since the metallic crystal has a continuous phonon spectrum (as opposed to a diatomic vibrational spectrum, for example), you get a continuous spectrum of light.

    Zz.
     
  5. Chi Meson

    Chi Meson 1,772
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    So it is correct to say incandescence it is due to the "kinetic molecular theory" model of vibrating atoms and molecules, and not at all due to electron transitions within the atoms and molecules?
     
  6. ZapperZ

    ZapperZ 30,437
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    Kinda. That last part is correct, but not the first part.

    Remember that a lattice in a solid does not involve just one or two or three, or even a few atoms/molecules. That's why I mentioned that this "vibration" is different than a molecular vibrational spectrum (which can have discrete lines). The phonon spectrum is the result of the collective vibration of the entire lattice.

    I think I've mentioned this before in the FAQ when we were dealing with light transport in solids. When atoms/molecules form a solid, they lose their "individuality", meaning most of the properties of the material are not due to the individual atoms, but rather due to the collective property of ALL the atoms. Nothing much of what we see is due to individual atom interaction. Rather, the collective property has form the solid's new energy bands that result in the well-known conduction and valence bands (for a typical band solid). These are what governs most of the solid's behavior.

    Zz.
     
  7. Chi Meson

    Chi Meson 1,772
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    Well that makes a lot more sense. It also means about 2/3 of the elementary texts are wrong in their explanation of incandescence. But what else is new?

    Just to be clear, and to make sure I'm not saying anything outright incorrect to my students: since E-M radiation is created by vibrating/accelerating charges, what could we identify as the charged objects producing light through incandescence?
     
  8. ZapperZ

    ZapperZ 30,437
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    If you consider the most simplified idea of a lattice as a series of + and - charges alternating with each other, i.e. +....-...+...-... etc... then there is a vibrational mode called the optical mode in which you can have, for example, the stretching and contraction of each individual dipoles, i.e. +..-.....+..-.....+..-.....+..-..... etc. You can think of it as a gazillion dipoles doing this contraction and expansion all over the solids, and in many different directions. The amplitude of the oscillation will give you an indication of the intensity of the emitted radiation.

    Zz.
     
  9. Chi Meson

    Chi Meson 1,772
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    I see no reason why my high school students can't understand that.

    Why is this kept a secret from elementary textbooks? Rhetorical question, no need to answer. Thanks so much, that clears up an irritation I've had for years.

    Now to work on that rash.
     
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