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Homework Help: Definition of Intensity

  1. Jul 3, 2006 #1
    Dear all,

    I have the next question: if we consider the classical definition of electromagnetic intensity it is given by [energy]/[time x surface^2]. At the same time intensity is related to the number of photons of the em radiation. So my question: given for example an intensity of 5W/[sec x cm^2] at energy Ep, how many photons at the same energy are emitted?

  2. jcsd
  3. Jul 3, 2006 #2


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    Staff Emeritus
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    Gold Member

    Depends on the frequency of the photons. Planck's relationship will give you the energy of a single photon, divide the power by the energy of a single photon to obtain the number of photons passing through the unit area per second.
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