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Photoelectric emission

  1. Apr 22, 2008 #1
    This is a question regarding photoelectric emission.
    When frequency of the radiation increases at a constant intensity, the number of electrons emitted per unit time decreases. Why is that so?

    p.s. I need an answer ample for Alevel purposes. Not too complex that is.
  2. jcsd
  3. Apr 22, 2008 #2
    1. what does it mean constant intensity?
    2. which is the energy of a single photon?

    When you answer those question you quite authomatically have the answer to your question.
  4. Apr 22, 2008 #3
    Can you please explain it?
    I mean no. of electrons per unit time is affected by changing intensity, which has been kept constant. And frequnecy usually affects max Ek,not the current?
    So why is it so?
  5. Apr 22, 2008 #4
    while taking it your way.
    1) Intensity is no. of photons arriving per unit area per unit time. So if intensity is constant, the no of electrons emitted per unit time should be const.
    2) Energy of a single photone is hf. So if frequency is increased, Ek should be affected, but not the no of electrons / time.

  6. Apr 22, 2008 #5


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    Staff: Mentor

    Usually, "intensity" of light means the amount of energy (e.g. joules) arriving per unit area per unit time.
  7. Apr 22, 2008 #6
    now im confused....someone with a comprehensive explanation to the original question please?
  8. Apr 22, 2008 #7


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    Staff Emeritus
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    Gold Member

    Referring to jt's comment:

    Intensity is a measure of the energy incident on a surface per unit area per unit time. And as you correctly say, the frequency is directly proportional to the energy per photon.

    Therefore, if the frequency of the light is increased, this means that the energy that each photon carries is increased. However, since the intensity or total energy per unit time and area remains constant, it follows that the number of photons incident on the surface must decrease since each photon carries more energy. Ergo, the number of photo-electrons liberated per unit time (current) will also decrease.

    I hope that clears things up.
  9. Apr 22, 2008 #8
    All you have to realize is that intensity is not the same as the number of photons. The number of electrons liberated depends on the number of photons, but if the intensity is the same while increasing the frequency, that means fewer photons and therefore fewer liberated electrons.
  10. Apr 22, 2008 #9
    thankyou everyone...
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