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Why do the electrons start moving even if the dim light (less intense) of a specific frequency falls on the material ?

Vinayak
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    Have you read http://en.wikipedia.org/wiki/Photoelectric_effect ? Especially the part about the work function? – Brandon Enright Nov 08 '14 at 21:07
  • I have read the part and I couldn't realize why it is so ? – Vinayak Nov 09 '14 at 01:54
  • It is experimental fact. Scientific models like light being composed of photons can help us make sense of it. Looking up "photons" may help. – BMS Nov 09 '14 at 04:46

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Because the light is absorbed mostly as single photons. Two-photon absorption is also possible, but it is a much rarer event.

A single photon carries energy $\hbar\omega$. If it is not sufficient to "lift" the electron to higher allowed energy, such a photon is scattered elastically, without absorption.

The problem is that the electron is not alone, it is attracted to the nucleus. And it is not a classical particle, but a standing wave. Standing waves have distinct frequencies( energies). Thus, there is an energy threshold for promoting an electron to higher levels.

Vladimir Kalitvianski
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  • Dim light means less intensity. the energy is given to a large area. Then how can an electron be ejected ? – Vinayak Nov 08 '14 at 16:18
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    @Vinayak: your statement the energy is given to a large area is not true. The whole point of the experiment is that light is exchanged between the EM wave and the metal in quanta. Quanta called "photons". The exchange of energy is localised, and a low intensity just means fewer photons per second. – John Rennie Nov 08 '14 at 16:28
  • similar to http://physics.stackexchange.com/questions/94369/why-photon-electron-energy-transfer-cant-occur-in-steps-or-does-it?rq=1 and http://physics.stackexchange.com/questions/73471/in-famous-einsteins-photoelectric-effect-why-does-intensity-of-light-doesnt-ra?lq=1 and http://physics.stackexchange.com/questions/68147/can-the-photoelectric-effect-be-explained-without-photons – ProfRob Nov 08 '14 at 16:41
  • @JohnRennie Intensity is given by energy incident per area per second...am i right ? – Vinayak Nov 08 '14 at 16:55
  • @Vinayak: It is right when the energy has a well defined continuous energy density and flux. In case of quantum effects, one has to keep in mind that the energy flux is due to flux of photons, particles, if you like. – Vladimir Kalitvianski Nov 08 '14 at 18:10
  • So how can low intense light eject an electron ? could you please explain more ? – Vinayak Nov 09 '14 at 01:56
  • @Vinayak: One photon carries enough energy to eject one electron. One photon is a very low intense light. – Vladimir Kalitvianski Nov 09 '14 at 06:43
  • @VladimirKalitvianski When this is allowed to fall on a large surface, wont they spread to other electrons – Vinayak Nov 10 '14 at 01:43
  • @Vinayak: No, they fall as particles, as droplets in a shower. – Vladimir Kalitvianski Nov 10 '14 at 07:26
  • @VladimirKalitvianski I couldnt get you. Then how is intensity related with area...I need more clarification on the packets of energy and their intensity – Vinayak Nov 11 '14 at 07:45
  • @Vinayak: The classical intensity is a continuous quantity. The continuity is a good thing for hight flux of individual particles. When the intensity gets low, it becomes discrete. In QM it is the probability to find a discrete particle on the area who is continuous, but particles fall one by one in uncorrelated order. Just as a heavy rain and a little rain. For the heavy rain the notion if intensity makes sense, but not for a little rain. – Vladimir Kalitvianski Nov 11 '14 at 09:50