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How do light bulbs really work

  1. Apr 15, 2009 #1
    I understand that the filament is heated by resistance and glows indandescently. What is the actual mechanism for the emission of light by incandescence? Are the electrons actually excited to higher energy levels or is something else going on?
  2. jcsd
  3. Apr 15, 2009 #2


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    Blackbody radiation. Filament gets hot. Filament cools down by releasing radiation:

    If the electrons were excited to higher energy levels (and then de-excited to emit photons) the light would only be at discrete wavelengths (e.g. in a sodium vapour lamp or a compact fluorescent lamp), and not continuous across multiple wavelengths as it is.
  4. Apr 15, 2009 #3


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    To add to that, you might think the filament getting to ~3-5,000K is ridiculous, but that is the case. Light bulbs are under vacuum however, that is why it doesn't blow up or anything :)
  5. Apr 15, 2009 #4
    Thank you for your replies. That is interesting about the vacuum thanks. What is the actual mechanism of black body emission?
  6. Apr 15, 2009 #5
    It's a great question and when it was first considered it kick started quantum theory. May I suggest that you start by googling the black body radiation curves.
  7. Apr 15, 2009 #6


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    My understanding, from Einstein's theory using his A and B coefficients, is that heat in a blackbody radiator is manifested in the kinetic energy of the particles, not in excited orbitals. The temperature of a medium is related to the average kinetic energy of the medium's atoms or particles. A photon carries with it a discrete amount of energy/momentum. When a photon strikes an atom, the atom absorbs the momentum and experiences an imparted impulse which moves the atom. These imparted impulses give rise to the kinetic energies that create the heat. So if we have a mass of oscillating/moving atoms, our blackbody radiator at a given temperature T, then they will spontaneously, and under stimulation, radiate some of their kinetic energies as photons. The probabilities at which these radiations occur (both stimulated and spontaneous) give rise to the Planck distribution. When photons are radiated from an atom, then the reverse happens from absorption, the atom loses a discrete amount of energy/momentum equal to the energy of the emitted photon.

    EDIT: Should make it clear in light of Division's post that Einstein's derivation was done before Quantum Mechanics described the atomic orbitals. Einstein related the kinetic energy gain from absorption to being countered by a retarding force. This last part here is not necessary to derive the Planck spectrum. So whether the energy that is being emitted is stored in excited orbitals or a kinetic energy of the atom is immaterial, the main assumption was that the energy emitted had to be done in discrete energy levels and the emissions could occur spontaneously or via stimulation.

    EDIT EDIT: Wait.. where did Division's post go... :eek: Ninjas! \shiftyeyes
    Last edited: Apr 15, 2009
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