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Some fundamental questions on light matter interactions

  1. Dec 21, 2012 #1
    Hi there,

    I've a few (probably very basic) questions about photon-electron (light-matter) interactions. Here we go:

    1) How can an electron "understand" that the frequency of incoming photon is equal to its excited state and so absorb that. Is there any "virtual" resonance between the electron and its excited level so a photon with the same resonance frequency can interact with the electron & electron's excited state system?

    2) Why (or how) do refractive index of a medium differs for light with different frequencies? It's probably related with energy conversation or something but I cannot get it.

    Mathematics of these questions are OK but I cannot understand these facts in a physical sense of manner.

    Thanks in advance.
  2. jcsd
  3. Dec 21, 2012 #2


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    Quantum mechanics. You can calculate it, but I don't think there is an intuitive model for this. Rabi oscillations are a bit like your description, but they have a different frequency.

    Polarization can vary with frequency, especially close to resonances in the material.
  4. Dec 21, 2012 #3
    Thank you for the answers mfb.

    Then, for the first question, in quantum mechanics we can describe what is going but cannot why or how it is going on, right?

  5. Dec 21, 2012 #4


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    Physics cannot answer "why". It can describe things on very fundamental levels, but all theories are just based on observations and made to describe those observations.
  6. Dec 21, 2012 #5


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    Physics has no answer to the ultimate "why", but it can reduce the number of independent "whys" that go unanswered! Saying it's "simply because of quantum mechanics" -- we can do better than that. :wink:
    Forget about the frequency of the photon and focus on the energies involved. A state with a well-defined energy evolves according to the Schrodinger equation with a well-defined frequency, but the states involved are not just the photon by itself, they are all electron + photon. And so the relevant frequency pertains to the combined energy.

    Consider two states: (a) electron in the ground state, plus photon going by, and (b) electron in the excited state by itself. If it's the "right" photon, (a) and (b) have the same total energy. As long as we ignore the coupling term, (a) and (b) are degenerate eigenstates and evolve independently of each other. The transition never takes place But if we now add the coupling term, (a) and (b) behave like a pair of coupled harmonic oscillators, and gradually evolve into one another. This happens only if the energies of (a) and (b) match, i.e. if and only if the "right" photon happens by.
  7. Dec 21, 2012 #6
    Hi 110010....
    the most intuitive explanation I've seen is via standing waves:

    [Wikipedia has this introductory explanation

  8. Dec 21, 2012 #7
  9. Dec 22, 2012 #8
    there is quite a general formula for determining it.Since classical and quantum mechanics essentially gives same result for it.it is amusing to note that classical description involves electron bounded to atom by a restoring force to get the refractive index.
  10. Jan 3, 2013 #9
    Thank you all for the replies!
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