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Can anyone help me with this problem regarding photoelectrons?

  1. Nov 24, 2004 #1

    I am new to this forum, doing A level physics, and have problems with photoelectrons and quanta etc....

    This is a question I would like help with:

    "Experiments on the photoelectric effect show that:

    -the kinetic energy of photoelectrons released depends upon the frequency of the incident light and not on its intensity

    -light below a certain threshold frequency cannot release photoelectrons

    How do these conclusions support a particle theory but not a wave theory of light?"

    I would apprecaite any input.

  2. jcsd
  3. Nov 25, 2004 #2
    You ask the most elementary question, but the philosophy behind the answer is at the root of the quantum revolution and still remains a problem for physics. Up until the 1905 publication of Einstein's explanation of the photoelectric effect, the wave picture of light of course was the only acceptable explanation for light theory. Even Planck's discovery in 1895 of the law for black-body radiation which contains the 'quantum of action' explaining the interaction of light with material bodies did not overthrow the wave picture. Planck did not refute the wave picture of free radiation which was assumed to be completely described by Maxwell's equations. He only suggested we modify the theory of light's interaction with material bodies. Planck did not realize that the field theory of classical radiation would have to ultimately be revised based on his equation.

    So along comes Einstein's theory of the photoelectric effect saying: E(max) = hv - P, where P is the work function of the electron escaping the surface. Then the simplest picture for this in Einstein's view, is that a light-quantum gives up all of its energy to an individual electron. That is, the photon must not be able to share its energy with several electrons such that increasing the number of photons will allow an electron to receive enough 'share' that it will escape the surface, as one might expect in a wave picture.

    Keep in mind, Einstein's theory of the photoelectric effect only surfaces if we consider the light in 'packets' coming in one at a time and at high frequency. Einstein's formulation as it is written only holds for hv >> kT. So the light-quantum postulate is only true where Wien's law holds for blackbody radiation. The remainder of the blackbody curve is explained by Jeans' Law, which is a wave theory. Later Einstein would state that we can not abandon Jean's law so we must modify our theory for 'complementarity'. Both particle and wave picture must be true and it is a matter of what experiment we run that allows us to view one feature more strongly than the other. This of course is what quantum theory would introduce but even more strongly stated many years later.

    Of course if you increase the light intensity so that the whole surface of the metal is strongly illuminated then you will measure light waves going in and electron waves coming out, even though you may think of the waves as composed of particles. So, yes you are correct, the photon theory does not refute the existence of waves, it just looks that way if you fire high frequency light pulses, at the lowest possible emission onto a metal surface and consider no other experiment.
  4. Nov 25, 2004 #3


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    Nick,you're a real danger...to physics itself and to its history in particular.Planck found his law at the end of October 1900,but he only made it public on December 14th 1900 in front of the members of the Prussian Academy of Sciences.
    It wouldn't hurt checking those physics books again,right??
  5. Nov 26, 2004 #4
    Sorry, I did pick that date from my memory....obviously flawed. Hopefully I did not mislead on the physics background itself (?), but let me know.

    ..reader beware.
  6. Nov 30, 2004 #5
    Photoelectric experiment

    I did the photoelectric experiment myself and plotted the graph. It was crude but was indeed what the textbook said. I want to reproduce this at home one day.
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