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Homework Help: Question about a derivation of jackson

  1. Jan 8, 2006 #1
    in the introduction to jackson's classical electrodynamics, he states that the number of photons incident to a 1 square centimeter area in 1 s for a 100W bulb (1 m away) is on the order of 10^15.

    also he gives a similar quantity for an FM antenna operating at 100W at a frequency of 10^8 Hz.

    what's the derivation for these numbers?

    I suspect that you need to find the intensity of the beams, then multiply by the 1 cm square to get the total energy - and then divide by the energy of 1 photon ( given by E = hf ).

    what I don't know is how to get the intensity.
  2. jcsd
  3. Jan 8, 2006 #2


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    The intensity is the power (given) divided by the surface area of a sphere 1 m in radius.
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