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Homework Help: Photon Intensity

  1. Sep 13, 2008 #1
    1. The problem statement, all variables and given/known data
    Supopse that a 100 W source radiates light of wavelength 600 nm in all directions and that the eye can detect this light if only 20 photons per second enter a dark adapted eye having a 7 mm diameter pupil. How far from the source can the light be detected under these conditions?

    2. Relevant equations
    [tex]E = hf = hc/\lambda[/tex]

    3. The attempt at a solution

    I am having a hard time finding a relationship between the 100 W source and a distance. At what rate do the photons die off?

    I can calculate the energy per photon: [tex]E = hc\ 600nm[/tex] but with that information, I still don't see a way factor distance in there.

    Anyone have some hints?

    how I can figure out at what rate it will dissipate over a distance.
  2. jcsd
  3. Sep 13, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi jesuslovesu! :smile:

    Hint: imagine a sphere of radius r round the source.

    How much energy falls each square metre of its surface? :smile:
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