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Need to understand about the photoelectric effect

  1. Aug 4, 2006 #1
    Hello my question involves what exactly is the frequency and intensity of the light. The E = hf applies to the photon particle aspect of light right? So where is f from, the wave aspect? Also increase f, which increases E makes the electrons eject faster in terms of kinetic energy right, but not increase in numbers. The intensity is what increases the number of electrons ejected but not speed? So is intensity part of the wave aspect? And how is it all related?
  2. jcsd
  3. Aug 4, 2006 #2


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    it is what is set by the device emitting it. intensity is the power of radiation crossing a perpendicular surface (to the wave direction) per unit area. power is the time rate of change of the energy passing that surface.

    f is determined by the color of the light. if it's white, there will be lot'sa different frequencies and lot'sa different electron energies.

    if the intensity is constant, the number of electrons (per unit time) ejected will have to decrease (but there would be more average energy per electron) if f increases.

    all other parameters held constant, yes. but if the frequency changes and intensity held constant, the number of electrons emitted per unit time will change (inversely).
  4. Aug 4, 2006 #3
    ok sounds like i'm on the right track. Just a little more, if frequency is constant. Increase in intensity the same as increase in power will eject more electrons. How does that work, more energy per length of time is distributed over each electrons, so more electrons ejected right. Now how does intensity connect with the wave aspect.

    Also with frequency, could you please clarify a little more about the whole color thing, I thought color was waves and not particle. How does colors tie to the particle aspect. And if intensity is constant, and frequency increases, which is more energy per electron, how does frequency affect the numbers when intensity is constant. You said it decrease?

    Oh by the way new question when this is tested in a circuit, why do u need a battery? Does the battery create a voltage that help electrons ejected go from cathode to anode bteween two plates like a capacitor? It will end up like a capacitor right
    Last edited: Aug 4, 2006
  5. Aug 5, 2006 #4


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    increase the number of photons (of the same energy) and more energy is crossing your surface therefore intensity increases.

    it's a "wave-particle duality". this is what it is all about. waves with frequencies of [itex]f[/itex] are associated with particles with energy of [itex]E = h f[/itex].

    if the intensity is constant and the frequency increases, the number of photons is less.

    you need an adjustable DC voltage source to oppose the electrons emitted and you increase the voltage until the current from emission just barely gets to zero. the electon charge times the voltage is the energy of the electron. that gives you the "E" and the color gives you the "f" and from enough points (different colors, different energies) you can get "h".
    Last edited: Aug 5, 2006
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