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Homework Help: Photon flux from a light source

  1. Jun 11, 2013 #1
    1. The problem statement, all variables and given/known data

    What is the photon flux (photons/ m2 s) at a distance of 1 km from a light source emitting 50W of radiation in the visible domain, with wavelength 6000 Angstroms.

    2. Relevant equations

    3. The attempt at a solution

    Energy of one photon = hc/λ = 3.32 x 10-19J.
    So, number of photons per second = 50/3.32 x 10-19 = 1.51 x 10-20.
    So, photon flux = 1.51 x 10-20/4π(1000)2 = 1.20 x 1013.

    Is this correct?
  2. jcsd
  3. Jun 11, 2013 #2


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    2017 Award

    Staff: Mentor

    1.51*10-20 photons per second? I think that is a typo.

    The approach is good, and the numbers look reasonable.
  4. Jun 11, 2013 #3
    Yeah! That is a typo!

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