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Photon's emmited by a 100w bulb.

  1. Sep 4, 2008 #1
    This is my question verbatim:

    "If 5% of the power of a 100w bulb is radiated in the visible spectrum, how many visible photons are radiated per second?"

    My work:

    Obviously, 5% of 100w is 5w, or obviously, 5 J/s.

    So the bulb is emanating 5 J of visble light per second.

    Now, I could easily solve this if I knew exactly what frequency (or wavelength) of visible light this bulb was emanating, but I don't. If I knew the frequency, I could simply use

    E (per photon) = hf where h is planck's constant.

    And then simply do 5 J / E (per photon) = # of photons.

    But I'm not given a frequency, and wavelength, or even what type of bulb this is.

    Any ideas?

    Mabye I should use the lowest and highest wavelengths of visible light and just give a range of photons...
  2. jcsd
  3. Sep 4, 2008 #2


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    Staff Emeritus
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    For a problem of this sort, just using the wavelength in the middle of the visible range (as an "average" visible photon) is probably sufficient.
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