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Why wave theory is wrong - A quick question

  1. Apr 5, 2012 #1
    With regards to the photoelectric effect the wave theory does not match up to observations of instantaneous emission of photoelectrons.

    Now my justification for this (not stated anywhere in my textbook or by my teacher) is that :

    Intensity is a measure of energy moving past unit area per unit time, correct ?

    Since a wave's intensity is proportional to amplitude2 there are periodic lows and highs in energy that a specific electron is receiving.

    Is this correct ?

    My one problem with this justification of mine is that I'm making use of amplitude when it is the displacement varying form one moment to the next. :/

    Could someone supply me with a good explanation ? Please, remember that I am an A Level/High School student.
  2. jcsd
  3. Apr 5, 2012 #2
    One easy way to argue against your idea is that there are waves (not in sound but in electromagnetism) that don't have highs and lows; it's called circular polarization. The wave goes through space like a corkscrew and at every point and every instant has a constant magnitude. These waves will also perform the photoelectric effect.
  4. Apr 5, 2012 #3
    I'll be honest and say I didn't full understand your explanation.

    Could you provide a suitable explanation for this observation ? Please take into account that I'm a high school student and this is an introductory course to Quantum Physics.
  5. Apr 5, 2012 #4
    There are two relevant types of radiation for the PEE, one with a frequency below the threshold (A) and one with a frequency at or above the threshold (B). If you crank up the intensity of A, you ARE transferring energy to the metal, but it's not the right KIND of energy. Where the quantum mechanics comes in is that the metal can only accept energy of certain frequencies in certain ways. Radiation A will go into vibrations and as you increase the intensity, you'll heat it up proportionally. If you use radiation B though, you're beyond the threshold and now you have a big enough size chuck of energy (energy of a single photon is proportional to the frequency) that the metal can accept it in a different way, by removing electrons.

    With regards to your idea, it's not clear to me what you mean.
  6. Apr 6, 2012 #5
    Elemis,according to quantum theory if a photon has a high enough frequency and therefore energy to release an electron then it transfers its energy to that electron with no significant delay.
    According to wave theory the electrons would be able to pick up energy continually from any incoming waves until eventually they have enough energy to escape.For low intensity illumination the time taken could be considerable.Of course wave theory fails here because it does not conform to the observations.
    You have already pointed out the problem with your justification.

    I think an explanation along the lines given above would be good enough to satisfy the A level examiners.
  7. Apr 6, 2012 #6
    Yes !! Perfect, just what I was looking for. Thank you Dadface !
  8. Apr 6, 2012 #7
  9. Apr 6, 2012 #8
    Well that is wave theory as it was understood at the end of the 19th century, when the relations between time(and therefore frequency) and energy were not so clear and the wave behaviour of electrons hadn't been observed experimentally yet.
    Wave theory as it is understood since 1926's Schrodinger equation of QM not only matches observations regarding PEE but explains it.
  10. Apr 6, 2012 #9
    An explanation in terms of A level was asked for.Schrodinger is not studied at A level but students are expected to know where classical wave theory breaks down when applied to photoelectricity.
    Last edited: Apr 6, 2012
  11. Apr 6, 2012 #10
    I guess this post is so superfluous to me as mine is to you. :wink:
  12. Apr 6, 2012 #11


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    A level Physics is at a really bad stage at the moment. In AQA they are told about Quarks before they are told about the electron volt. How daft is that?
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