Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Conceptual question on black bodies

  1. Sep 9, 2010 #1


    User Avatar
    Gold Member

    1. The problem statement, all variables and given/known data
    A piece of metal turns red when at 1100K while a piece of quartz doesn't "shine" (i.e. emits in the visible range I guess) at this temperature. Explain this phenomenon knowing that quartz is transparent to visible light.

    2. Relevant equations

    3. The attempt at a solution
    This question blows my mind. A first reaction would be to say that the quartz doesn't behaves like a black body since it doesn't absorbs (in the sense of a black body) visible light. This is however totally false in the sense that ice is also transparent and has an emissivity of 0.97 which is almost a black body (in fact I guess it absorbs any other light with different frequencies than light in the visible spectra).
    Also I don't know the theory of transparent materials. I know they absorbs light and then re emit it with the same wavelength and a perfectly transparent wouldn't absorb any photon (so a white body?), however it wouldn't reflect either, or few.
    I'm just clueless. If you have any idea, feel free to share.
  2. jcsd
  3. Sep 9, 2010 #2


    User Avatar
    Homework Helper

    Probably the availability of the free electrons is the reason of red hot metal. When the photons fall on the metal, the electrons absorb them. Since the electrons are free to move, this energy is dissipated in the metal, increasing its temperature. In the quartz, the electrons are tightly bound. So the energy absorbed by them is mostly re emitted. It is the cause of transparency. Quartz has a resonance phenomenon due to which it blocks certain frequencies.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook