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In what direction is thermal radiation emitted?

  1. Feb 3, 2009 #1
    It is well known that thermal vibrations of molecules produce thermal radiation or photons (infrared and visible): but in what directions?
    I can understand in a gas or liquid the molecules will be rotating in all directions somewhat uniformly and, thus, photon emission will be uniform in all directions (diffuse). But in a solid or crystal, with the molecules being restrained somewhat, would not the photon emission be in preferred directions?
    Specifically, construct a simple capacitor - two metal plates separated by a dielectric material. The dielectric material will emit thermal photons in a diffuse manner from its surfaces not covered by the metal plates (i.e. from the edges of the capacitor). If a low DC voltage ( up to 100 volts) is applied to the plates, will the dielectric molecules rotate to align with the electric field and, thus, emit photons in a more concentrated manner,hopefully parallel to the metal plates? And what dielectric would be best for this result? Thanks
  2. jcsd
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