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On photoelectric effect

  1. May 30, 2007 #1
    Below is my understanding of photoelectric effect, please correct me if I am wrong.

    It is an quantum electronic effect in which matter emits electrons after receiving energy in the form of EM waves. E.g. imparting X-rays on matter.
    So if we impart a high frequecncy wave (say f1) on a metal body (same thing that happens to antenna), it would emitt electrons???
    Here I assume that f1 is far greater than threshold frequency required to make electron escape from crystalline structure.

    Then why doesnt my cell phone antenna emitt electrons...

    It is an quantum electronic effect in which matter emits electrons after receiving energy in the form of EM waves. E.g. imparting X-rays on matter.
    So if we impart a high frequency wave (say f1) on a metallic body, would it emitt electrons???
    [Here I assume that f1 is far greater than threshold frequency required to make electron escape from crystalline structure.]

    Then why doesnt my cell phone antenna emitt electrons...????

    There could be only one reason that cell phone frequency is well below threshold frequency...Because even with lowest possible intensity (only one photon at a time), frequency which is above threshold would make atleast one electron escape.


    Please correct if I am wrong, thanks in anticipation.
     
  2. jcsd
  3. May 30, 2007 #2

    malawi_glenn

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    The antenna emitts electromagnetic waves due to the oscillating filed inside and around it. You do not impart light (EM waves) on the antenna in the same manner as you impart light on metal surface so it will knock out electrons.

    If you apply an alternate current in a circiut (the circut that gives you the antenna if you have studied electronics), the moving electrons will create a magnetic field, and the magnetic field will interact with the electric field from the electrons and so on, you get a electro magnetic wave from the antenna.
     
  4. May 30, 2007 #3
    jainabhs,

    You are right that you cell phone is "irradiated" by photons emitted by the antenna.
    But these photons are far below the treshold.

    Photons from the visible and UV spectrum are typically in the petaHertz domain of frequency (10 to the power 15, see http://en.wikipedia.org/wiki/Tera), and their wavelength is typically micrometers.

    Photons from your cell phone are typically in the gigaHertz domain of frequency (10 to the power 9) and their wavelength are typically 3 meters.

    Therefore, the energy of "visible or UV" photons are typically one million times more energetic than gigaHertz photons.

    The photoelectric effect cannot be observed with gigaHertz frequencies.

    Does that mean that photons in the gigaHertz range of frequency are not observable: absolutely no.
    Photons from even lower energies can be observed.
    There are numerous experiments where such photons are observed individually.
    One of my latest reading in a magazine was about such an experiment.
    One single photon was trapped in (cooled) cavity. The interaction of this photon with an atom passing through the cavity produced some signals that could show the existence of this unique photon in the cavity But the purpose of this experiment was even more interresting .... (see http://physicsweb.org/articles/news/11/3/9)


    See: http://en.wikipedia.org/wiki/Electromagnetic_spectrum for an overview of the Electromagnetic spectrum
     
    Last edited: May 30, 2007
  5. May 30, 2007 #4

    Claude Bile

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    The energy of the incoming photon must exceed the work function to liberate an electron. Photons at GHz frequencies simply have insufficient energy to trigger this process.

    Claude.
     
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