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The photoelectric effect

  1. May 22, 2004 #1
    when UV light is shone upon a metal, can electrons be ejected from the metal or does this only occur if there is another force in play (eg a potential difference to sweep the elctrons away)

    also, if electrons (at their base energy level) only accept a specific frequency of UV light (which varies from element to element) [<correct me if i wrong here], then how come when you increase the frequency of UV light on a surface, we observe an increase in an emitted electrons total energy (ie, the electrons still absorbed the the EM radiation)
     
  2. jcsd
  3. May 23, 2004 #2
    Electrons can be ejected, but without an accelerating PD they won't travel very far!

    Einsteins photoelectric equation tells us that any 'spare' energy left over after ionisation is given as Kinetic energy. Hence, as the energy of incident photons increases, so does the emitted electrons KE.

    Each metal has a so called 'work function' which is the energy needed to eject an electron, so yes, each element will have a different value.
     
  4. May 24, 2004 #3
    however i was also under the impression (from my teacher) that individual electrons can only accept a specific frequency of radiation at different quantum levels (ie, at their base level they may, for example, only be able to accept one frequency of UV light). How does this explain the fact that, whether it be a high or low frequency radiation shining on them (above the threshold), we still observe them being ejected from the metal (with different KE of course)
     
    Last edited: May 24, 2004
  5. May 24, 2004 #4
    ummm this effect happens only one photon (light particle, in this case light behave like a particle, but although overall it's a wave) <--- help me explaine more

    only one photon can knock one electron from the metal. energy of photon is "qutan" that is in packets or different level but not continouse building up.

    you either use one photon kick out one elctron but not two half photon.

    different frequency can give different energy to phton enable to kick out one electron.

    it also define ke= hf-W w=work force for each metal. h = contant f= frequency.
     
  6. May 24, 2004 #5
    Accepting only specific energies (or frequencies) is true for excitation. Once you have enough energy to ionize the electron, any energy can be absorbed. The free electron energies are not quantized, only the bound state energies.
     
  7. May 24, 2004 #6

    ZapperZ

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    As swansont has mentioned, the conduction electrons (which are involved in most elementary photoemission experiments) has a continuous energy band. If the photon you use has an energy just barely equal to the work function, then you will, in principle, get only photoelectrons with a very narrow range of energy. However, if you can do that, I have an accelerator photoinjector that I want you to work at! :)

    Most of the time, your photons are at some energy significantly above the work function. What happens in this case is that a single photon can cause the emission of electron from the top of the conduction band to an energy [tex]h\nu - \Phi[/tex] below the top of the conduction band. This whole range of the conduction band is available to the photon to excite an electron into the vacuum state.

    So the energy spectrum of the photoelectrons is not really a function of simple photoelectric effect experiment in the Einstein model, but rather a direct reflection of the band structure of the material being used. That is why such an experiment, nowadays called photoemission, is such a valuable technique in studying the band properties of materials.

    Zz.
     
    Last edited: May 24, 2004
  8. May 24, 2004 #7
    I've got some itches concerning the analysis of the photoelectric effect...

    First,
    One very good thing about this experiment is that the difference between classical and QM prediction is very distinct and fairly easy to understand. It's always like .. "If light was a wave, then this and this would happen.. and if light was a particle, this and this would happen .." ... But there is a hidden assumption here that's never presented, namely that the waves are sinusodial. Am I wrong in saying that classical physics had no model for atomic EM emission? So what were they basing this assumption on? What natural nano-wavelength source has charges oscillating in such a specific way as to produce multiple continuous sinusodial waves with different wavelengths ? - answer: None. It's not possible.

    Second,
    In classical physics the process of emission and absorption isn't as easy as it's pictured in QM. A wave doesn't simply hit a particle, transfer all it's energy, and then disappear. What % of the total energy a wave transferes is very dependant on the "size difference" between the wave and the particle. Ironically, Heisenbergs thought experiment with the microscope explains this well.
    Radio is another example where this wave/receiver relationship is important.
     
  9. May 25, 2004 #8
    *scratch* *scratch*
     
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