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Homework Help: Photon flux in Modern Physics

  1. Feb 4, 2016 #1
    1. The problem statement, all variables and given/known data
    It's in attachment

    2. Relevant equations

    E=hf. Where h= planks constant and f= frequency
    3. The attempt at a solution
    I know that the flux of a photon is (# of photons)/(sec m^2)
    I don't know the number of photos but I do know the frequency and power.

    E=(6.6261*10^(-34)) J•s * (100 MHz)=6.621*10^-32
    I know that it's traveling 4 light years away.


    What equation am I supposed to use and how do I find the number of photon and how big in diameter would the recover/detector have to be. I don't want the answer just help understanding the problem and what I'm doing wrong. Or am I on the right track

    Thanks in advance

    Attached Files:

    Last edited: Feb 4, 2016
  2. jcsd
  3. Feb 4, 2016 #2


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    Staff: Mentor

    The image has a wrong orientation.

    You know the energy per photon, and the power. Can you get the photons per second, or per image frame?
    To capture all photons in a distance of 4 light years, aliens would have to construct a sphere with a radius of 4 light years. They don't need all the photons, of course, but which fraction do they need?
  4. Feb 4, 2016 #3
    I know the
    energy is 6.621*10^-32
    power is 100Kw 24 frames per second.

    what equation should i use to relate them
  5. Feb 4, 2016 #4
    I can do the power divided by the photon energy to get the number of photons arriving per second

    100 KW/ 6.21*10^-32
  6. Feb 4, 2016 #5


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    Staff: Mentor

    As every second has 24 frames, how many photons are there per frame?

    All those steps all don't need fancy physics.
  7. Feb 4, 2016 #6
    power divided by / ( the photon energy * 24) to get the number of photons per frame
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