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B Photoelectric effect

  1. Aug 10, 2016 #1
    Hey I have a doubt that when on a metal surface photon falls then electrons are ripped off if the photon has frequency equal to or more than threshold frequency but photon falling on electron has a downward momentum and elctron rips off in a upward momentum....how is it possible according to law of conservation of linear momebtum?
  2. jcsd
  3. Aug 10, 2016 #2


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    hi there

    firstly, the electrons are not "ripped off". They gain energy from the interaction of the absorption of the photons.
    When an electron in the material material absorbs the energy of one photon and acquires more energy than the electron binding energy to hold it within that atom structure, it is ejected/emitted.

  4. Aug 10, 2016 #3


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    The conservation of momentum (or explaining how and where it is involved) is not really an issue here. The total momentum of the whole measuring apparatus would need to be conbnsidered. The incident photons carry momentum which will turn up as an increase in momentum of the equipment or even the Earth. The momentum of the original photons also needs to be considered with regard with where they were originally produced.
    It's easier to ignore all this as the mass of the equipment is so much greater than that of the photons.
    Don't lose any sleep over this one.
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