- #1
mmoverman
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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?
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?