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Black Body / Emission difference ?

  1. Jul 21, 2007 #1
    If I make a ball out of calcium; drill a little hole in it; then heat it up in a suitable [vacuum] environment; I see black body radiation thru the little hole, right? At least up to the melting point of calcium ?? I mean, black body radiation is independent of material, right ?

    Yet the "emission spectrum" of calcium is dark brilliant red. That is what you see if you put calcium in a flame.

    So when do you get black body radiation? Only below "emission spectra" temperatures? How come I found a chart of black body radiation for 5500 degrees Kelvin?? [ Wikipedia I believe]

    Related question: the NASA COBE experiment was measuring black body radiation, right?
     
  2. jcsd
  3. Jul 21, 2007 #2
    Good question. The answer is that black body radiation comes from a completely different mechanism than the one that produces emission spectra. Black body radiation comes from the molecular motion of material, which, as you point out, is independent of the atomic make-up of the material. The emission spectrum of a material comes from the electrons in the atoms falling from one quantum state to another, and as such, is very dependent on the specific atoms that make material. That's why emission spectra consist of sharp lines - they correspond to the energy differences between the specific allowed quantum energy levels for the electrons. The black body spectrum, on the other hand, is continuous and strictly speaking, covers all temperatures, not just a limited set.

    And yes, COBE was measuring the background radiation of the universe as a whole (neglecting those little clumpy things like stars, galaxies, cluster, etc. :wink: ), which turned out to match the predicted black body curve to a remarkable degree of accuracy.
     
    Last edited: Jul 21, 2007
  4. Jul 21, 2007 #3
    A big YES! three times. Thank you for confirming my understanding that:

    1. Black body radiation is independent of material

    2. Emission spectra ARE dependent on material becuase they are generated by jumps between electron shells

    3. Black body radiation, in stark contrast to emission spectra, is continuous with respect to wavelengths.

    But these three facts only intensify my puzzlement. Wikipedia definitely has an article on black body radiation showing a curve for black body radiation at 5000 degrees Kelvin, and the ENTIRE black body spectrum, including infrared as you mention, was THE data inspiring quantum menmechanics. Pretty important data. So how do you get it ?? 5000 degrees Kelvin is the temp of a very hot flame which should generate EMISSION spectra, no?
     
  5. Jul 21, 2007 #4

    xez

    User Avatar

    Well IMHO the answer is 'all of the above'
    that blackbody radiation is according to the Wien
    distribution curve and is not so dependent on material
    (though materials certainly do have emissivity et. al. that
    varies).

    There WOULD be atomic orbital emission spectra produced
    according to a probability that depended on the average
    atomic energy in the boltzmann distribution and the
    statistics governing einstein transition probabilities for
    the orbitals involved.

    However you almost always 'see' emission spectra
    produced by isolated small amounts of matter -- ions,
    atoms, molecules, low density plasma sources, et. al.
    This is simply because there are effects like the doppler
    effect and raman scattering and so on that cause spectral
    line broadening as a result of higher 'pressure' and
    velocity within the material so that an element as a
    low density plasma will emit very narrow lines, the same
    element as a solid or high pressure gas tends to emit a
    much broader spectrum since the line-width becomes
    wider.

    Contrast the emission spectra of a low-pressure sodium
    lamp with a high-pressure sodium
    (hated among astronomers for this very reason) lamp.

    The pressure and doppler broadening effects are why
    we see a basically continuous spectrum from the solar
    photosphere even though it's mostly
    ionized Hydrogen/Helium plasma; the pressure and
    temperature are so high the lines are very broad, and
    basically it has become a blackbody radiation source
    at the equivalent temperature.

    The solar corona is cooler and at lower density, so from it
    we do see more pronounced sharp spectral lines.

    At intermediate temperatures/pressures there's no
    conflict between having quantum emission events and
    blackbody spectra, however, it doesn't have to be one or
    the other. I think the emission 'lines' are transitions that
    are just a subset of the overall rules of E/M and QED that
    show the various ways and probabilities that a system
    can radiate.

    Also typically in solids you might have a different
    configuration of valence / outer orbitals than you might
    have in the same element were it mono-atomic gas or
    ionized. Since some of the outer electrons
    are participating in some kind of ionic/covalent bonding
    in solid crystals/molecules, the emission spectra will be
    different in those cases. There will still be discrete
    spectra with discrete probabilities, but they'll differ in
    respect to certain shells and transitions for those electrons
    that are participating in a bonding process.
     
  6. Jul 21, 2007 #5
    Whew! Thnx.
     
  7. Jul 21, 2007 #6
    yeah ..... what HE said.
     
  8. Jul 26, 2007 #7
    A related question(s): 1) A drop in electron energy level emits photons; is the energy level rise caused only by photon absorption, or can that energy come from something else, like heat? 2) I understand that ionization does not work the same way -- ala photon emission/absorption: correct? 3) I understand infrared absorption shows up inside the molecule as increased bond (translinear or bending) or rotation energy: correct? That energy can be given up through emission at the same discrete wavelength (line), or can be passed as kinetic to another molecule in a collision: correct? Can/does it transfer (or add to) the initial molecule's "Center-of-Mass" kinetic temperature?

    I'm just trying to get my arms around the details of emission/absorption. Thanks for any help
     
  9. Jul 30, 2007 #8

    Claude Bile

    User Avatar
    Science Advisor

    From the sun :wink:?

    To RodB; 1) Yes there are many ways you can 'excite' an atom into a higher orbital. Apart from exciting one optically or thermally as you have mentioned, one can also excite an atom electronically, chemically and mechanically. Don't regard this as a complete list, because I'm sure I've left a couple out. 2) I disagree, I think one can ionise something without using photons - lightning for example is a result of ionisation due to electrostatic force between a cloud and ground ripping electrons away from their parent atoms. 3) Yes, many gas lasers such as Helium-Neon and Carbon Dioxide lasers depend on these types of interactions.

    Claude.
     
  10. Oct 2, 2007 #9
    Another follow-up: it's clearly implied here that gases can and do emit blackbody-type radiation -- Do they? I've been having a debate on a climate blog -- some say yes, some say yes but only under very high pressure (I asked what's the pressure of the Universe background radiation), some say definitely not (though some here accept plasma radiating). All are credible folk. Can antine shed some light on thisd? Thanks.
     
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