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Intensity, photons, human eye

  1. Apr 4, 2009 #1
    hi there. there's an example in the book, but i'm having a little trouble here.

    1. The problem statement, all variables and given/known data
    Assume that the human eye can pick up as few as 9 photons/s in the visible range. Based on this, estimate the intensity of the dimmest star that can be detected by a night-adapted eye. What is the ratio of this intensity to the intensity of noon sunlight, some 1400 W/m2? This large intensity range means that the eye is indeed a very adaptable instrument.
    Answer format = (intensity of 9 photons/s / intensity of noon sun)

    use 3mm for the radius of the pupil.
    use 550 nm for wavelength.

    2. Relevant equations
    I=N*E, N is the number of photons/(m^2*s)

    3. The attempt at a solution

    So I want the intensity, which is N * E, which is (N * h * c) / (Pi * r^2 * lambda). I get the intensity of the dimmest star on the human eye as 1.14592 * 10^(-13).

    With the given intensity of noon sunlight (do I need to adjust this for the area of the pupil?...), I divide it. 1.14592*10^(-13) / 1440 = 7.95775*10^(-17).

    but this is wrong, so... either I was supposed to adjust the noon intensity, or I've made one or several other mistakes.

    I'd appreciate any insights or tips.


  2. jcsd
  3. Apr 3, 2010 #2
    your equation is correct
    check ur calculations^_^
  4. Apr 3, 2010 #3


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    Science Advisor

    I got the same numbers. Why do you think it's wrong? What do you think the right answer is?
  5. Apr 3, 2010 #4
    I got 1.150429803e-13 for the intensity of the star

    divide it by 1400, the final answer should be 8.217e-17
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