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Homework Help: Power of LED and human eye

  1. Sep 21, 2012 #1
    1. The problem statement, all variables and given/known data
    In ideal conditions human eye can feel something if 100 photons with wavelengh of 5500A fall directly tu human eye in one seconds time.LED(lambda=5500A) is placed 5m away from human eye.

    2. Relevant equations

    Calculate the power of LED to achieve that the human eye can see it?

    3. The attempt at a solution
  2. jcsd
  3. Sep 21, 2012 #2
    What is your difficulty?
  4. Sep 21, 2012 #3
    I even dont know where to start here.
    Some kind of idea would be helpfull.
  5. Sep 21, 2012 #4
    Each photon carries energy. How much?
  6. Sep 21, 2012 #5
    I know that much
    E=hv and v=c/lambda so E=hc/lambda
  7. Sep 21, 2012 #6
    So you should be able to solve the problem.
  8. Sep 21, 2012 #7
    You also need to know how many photons miss or hit the pupil. Perhaps they all bit because of lenses. Or not.
  9. Sep 23, 2012 #8
    Ok I know the energy which is needed to feel something.
    I assume that all hit the pupil and that diameter of an eye is d=15mm.
    So i can calculate the solid angle.

    But still how can I calculate power of LED ?
  10. Sep 23, 2012 #9
    I think you can assume that the LED radiates in all directions. You have a particular solid angle where particular energy must be radiated to be be detectable. What else do you need?
  11. Sep 23, 2012 #10
    I think i got it.
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