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Incandescence vs. luminescence

  1. Feb 28, 2010 #1
    I am a both a physics and chemistry teacher and I have been frustrated in my attempts to give my high school students a clear and unified (but level appropriate) definition and understanding of the concepts of incandescence (sometimes referred to as "hot body" radiation) and luminescence (sometimes referred to as "cold body" radiation.

    Having looked at sources and texts of different levels, I feel that there is often ambiguity introduced in at least the following ways: "light" as defined as the entire EM spectrum vs. visible light, molecular vibration (with springs for bonds) with a "natural frequency" as an emitter/absorber such as characterized by an IR spectrometer vs. electron transitions, and classical vs. QM explanations . And many explanations are clearly wrong.

    I'd like to put out what I'm getting out of all of this and put it out there for comment/corrections.

    I seem to be reading that luminescence should be viewed as arising from specific electron transitions that are driven by energies other than heat. As such, we could expect a line spectrum or even monochromatic light. Examples would be chemiluminescence (chemical energy) and a gas discharge tube (electrical energy, though that is mediated by collisions with the ionized gas and free electrons which I think could then be referred to as heat). Perhaps LED's could even be considered an example of luminescence. Also included would be examples were the absorbed energy comes in the form of photons such as fluorescence and phosphorescence. However, this then requires an explanation of the continuous spectra produced by phosphors such as in fluorescent light bulbs. I'm not sure what character the light from a process like triboluminescence has other than I've read that it is akin to lighting which I guess then makes that a mystery for me as well.

    Then there is incandescence. Light emission (specifically visible?) driven by "heat" which itself may require clarification in this context. I like to darken a room and show my students a kitchen hot plate with a coil. I explain that it is emitting radiation all of the time (focusing on the IR). I assume that with the complexity of the molecular and intermolecular forces, the various "natural frequencies" as described above are so numerous and close to each other that the spectrum is "smudged" (as Hewitt) producing a continuous spectrum in the IR range. (One point being that in a sense, there is no such thing as a truly continuous spectrum.) But then as a the temperature goes up, there is the emergence of visible light. Presumably, whole atoms and molecules are too massive to vibrate with the requisite frequencies for visible light, therefore we at some point must shift to electron vibrations/transitions. Once again, the particle interactions produce a "continuous "spectrum. (Also implied is a shift from a classical view to a QM view). I'd like to avoid going too deeply into black cavity radiation the "ultraviolet catastrophe" and even quantizing heat as phonons or whatever.

    A glowing gas in a ceramics kiln whose light appears to be no different from the surrounding solids is generally described as being incandescent. But how about the colored light emitted by vaporized metals we see in the "flame test". This is driven by heat but surely arises from electron transitions. Incandescence or luminescence?

    And how about the faint blue light at the bottom of a candle flame. I've seen it explained as arising from carbon pair fragments. Presumably this emission is driven by heat but does the light come from extremely fast molecular vibrations of from an electron transition? Is it monochromatic? Should this be called incandescence?
     
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  3. Mar 1, 2010 #2

    Andy Resnick

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    Something to keep in mind is that emission, of any sort, from a multiatomic body (condensed matter) has a spectrum significantly broadened as compared to that of a gas or single atom. There can indeed be continuous spectra- for various reasons.

    So incandescence: light emission from heat. Certainly the various spectral ranges (visible, IR, etc) can correspond to transitions, but blackbody radiation does not require a material body: it's the spectral distribution of light in thermodynamic equilibrium with a well-defined temperature. Combusting systems have a lot of chemical reactions occurring so the specifics are complicated, but that's why flame tests are useful.

    http://en.wikipedia.org/wiki/Flame

    Your electric burner has the spectrum it does because there are conduction electrons that are basically free to move around and collide with other atoms; the spectrum approaches that of a blackbody to some degree due to this 'thermalization' of the conduction electrons.

    Luminescence, as I understand it, involves specific transitions between bound states. Fluorescence, for example. Phosphorescence (the reason for the broad-band output of fluorescent lights) involves forbidden transitions- they are long-lived, leading to broader output spectra. LEDs and superluminescent diodes are fluorescent- they are driven below the lasing threshold. White LEDs have a phosphorescent coating.

    I'm not sure on the mechanism for triboluminescence, if it's conceptually different from or similar to lightning, for example.

    Does this help? This is a broad topic....
     
  4. Mar 1, 2010 #3
  5. Mar 1, 2010 #4
    Yes it is a broad topic in the sense that I am trying to unify a number of different phenomena and methods of producing EM radiation. The wikipedia article is very good. I've been trying to find the blue light spectrum from a flame. I have actually researched this topic quite a bit but never thought to put "flame" in the wikipedia search.

    I can see by some other threads listed below that there are some issues of semantics involved (some quite passionate!) There is also the uncomfortable tendancy to deal with low energy situations in terms of vibrations and waves while higher energy situations evolve into bound states and photons. Not to mention that as I understand it, Planck's black body radiation derivation says nothing about the material, but instead upon allowed modes of vibration (EM standing waves).

    A couple of issues remain unclear for me:

    Does a material have to be stimulated by light in order to be called fluorescence or phosphorescence?

    In the burner, is it reasonable to say that the radiation emitted at room temperature comes from the lattice stucture but as the temperature rises, that electrons are somehow recruited into the process? I'm assumiing from your reply that the thermal electrons are responsible for the visible light.

    I'll probably think of more questions, but thanks for your help.
     
  6. Mar 1, 2010 #5
    I love this quote from the Wikipedia article on black body's.

    "Due to the rapid fall-off of emitted photons with decreasing energy, a black body at room temperature (300 K) with 1 m2 of surface area emits a visible photon every thousand years or so, which is negligible for most purposes."
     
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