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Homework Help: Photons emitted by a blackbody

  1. Aug 26, 2007 #1
    1. The problem statement, all variables and given/known data
    Consider a 75 Watt light bulb and an 850 Watt microwave. If the wavelengths of the radiation they emit are 500nm and 150nm, respectively, estimate teh number of photons they emit per second. Are teh quantum effects important in them?

    2. Relevant equations

    [tex] M(T) = \sigma T^4 [/tex] Now this gives me the watts per meter^2. But I dont know the meter^2. (This doesnt make sense to me because the watts/meter^2 should be a function of the distance, farther out --> less watts/meter^2, but this function says nothing of distance...)

    3. The attempt at a solution
    Dont know where to start!

    edit- My thoughts are I need to get teh Joules per second emitted from the object. I can then turn this into photons per second using the relationship [tex] E = h\nu [/tex]

    Thx for any help!
    Last edited: Aug 26, 2007
  2. jcsd
  3. Aug 26, 2007 #2


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    Your second thoughts are correct, you don't need to consider the blackbody (which the microwave isn't). Just use E=hv , or E = c/lambda to get the energy per photon then the power you have been given.
    ps I think you have misread the wavelength for the microwave it should probably be a few cm, 150nm is UV.
  4. Aug 26, 2007 #3
    Yes you are right, the microwave is 150 mm.

    Now I have the answers in photons/second. The question "Are the quantum effects important in them?". I have to figure out if the quantized nature of the radiation is significant. hmm.... Im thinking its no, but I have no reason for that. I dont see and quantized values in the equations I wrote...
    Last edited: Aug 26, 2007
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