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Does the color of a hot body depend on its composition ?

  1. Jan 30, 2012 #1
    Hi
    Does the color of a hot body depend on its material composition ? For eg. will bodies made of steel, Stainless steel, Ti, Cu etc. all look red in color if the temperature is ~ 550 deg C ?

    TIA
     
  2. jcsd
  3. Jan 30, 2012 #2

    Simon Bridge

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    "material" is quite broad.

    In general, "sort of".
    Atoms have a characteristic spectrum with dominant lines.
    It depends on their electronic structure.
    http://en.wikipedia.org/wiki/Emission_spectrum

    The blackbody spectrum occurs under specific conditions.
    But those conditions are pretty common.
    http://outreach.atnf.csiro.au/education/senior/astrophysics/spectroscopyhow.html [Broken]

    Your examples are not going to be all that different at that temp though - heat them up and see. Metals share their outer electrons across the whole solid as a kind of big tub of charge so they all have very similar spectra under heating. Also heavy elements have more options to emmit from, allowing thermal emmissions ot spread over a wider range of wavelengths.
    http://911research.wtc7.net/cache/wtc/analysis/fires/metcolor.htm
    Compare the emmission spectrum of H with Fe in the wikipedia article.

    You normally bring out the characteristic colors by EM "pumping" as in florescent lights ... hydrogen is pink, sodium is orange, and mercury is blue-grey.

    The bottom line is that you can use the color to work out the temperature.
     
    Last edited by a moderator: May 5, 2017
  4. Jan 30, 2012 #3

    sophiecentaur

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    If you can get a body hot enough to radiate significantly at optical frequencies then I think its reflectivity of different frequencies (it's apparent colour) could have an effect on the spectrum of radiation it emits (i.e. radiated colour). After all, emissivity and absorptivity go together, so if the body 'looks green' it will be absorbing reds and blues and reflecting greens. So, if the body were at a 'white hot' temperature reds and blues would then be radiated better than green (making it 'glow' magenta more than white). HOWEVER, at that temperature, the energy levels in the surface molecules would be much modified and their interaction with different frequencies of em would probably be changed. So what I say may not apply at all.
     
  5. Jan 30, 2012 #4

    Andy Resnick

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    It's probably not what you are thinking of, but there are thermochromic materials that show a wide variation in color with temperature.

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

    Aside from that, if the refractive index (and thus the emissivity) varies with temperature, the color of an object will also change with temperature.
     
  6. Jan 30, 2012 #5

    sophiecentaur

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    Got the T shirt! Way back.

    Actually, there is mention of heating metals, earlier. The change in surface (reflective) colour of metals (tool steel in particular) when you heat them is a great indication of their temperature and is often used when hardening and tempering etc. but that's at lower than 'red heat'.
     
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