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Photoelectric effect

  1. Nov 19, 2012 #1
    a light with frequency 'v' can't excite electrons from a metal surface if the threshold frequency of the metal is greater than the frequency of the wave( the light,electromagnetic radiation)so why don't the energies of the photons add up together and then excite the electron in the surface of the metal (mean even a very intense light with the same frequency can't do it if it has less frequency intense light means lots of photons cant this photons co-operate?)
  2. jcsd
  3. Nov 19, 2012 #2
    You got it right. Kinetic energy of emitted electron is function of frequency and not intensity.
    For very high intensity of light, it is possible.

    You can throw ball up in air only up to certain height. Call your friend to help you and you together can not throw it higher (without using any tool). Friend will be useful to throw heavier ball, but then it is a different attribute.
  4. Nov 19, 2012 #3


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    It is possible. Look up "multiphoton photoemission". This is an important technique in studying the energetics of Rydberg-type states. Note that one must use a light source that has a very high photon density. This is because the excited state has a very short lifetime, and the second photon that comes in must interact with that excited electron before it decays back to the conduction band. The probability of this happening (what we call the "cross-section") is quite low and thus, one must use a light source with a very high photon density per unit area to increase the probability of it happening. For ordinary light sources, this probability is practically zero. That's why the ordinary photoelectric effect is commonly observed.

    What this means is that the ordinary photoelectric effect is a single-photon photoemission phenomenon.

    BTW, note also that there's another way to cause the emission of a photoelectron even using a light with energy lower than the work function. One can apply an external electric field to the surface, thus lowering the effective work function. This is usually known as the Schottky effect.

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