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The ejecetion of an electron from a metal occurs when a photon from a sufficiently high energy light is made to fall on metal. My question is that isn't wave capable of doing the same? Is it so that the wave can only librate the electron but with the help of photon the electron can be librated and also it can have kinetic energy(due to the mass of photon)?

Mad Dawg
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This answer is simplified and rather provides the gist.

Your question is exactly pointing to the characteristics of the photoelectric effect that made people scratching their heads. When light is considered a wave then energy would be transferred continuously to the electron. In effect, the electrons are released as soon as their binding energy is exceeded. Accumulating the energy would take some time, thus electrons are released some time after the light is switched on. This time depends on the intensity of the light.

Nothing of that is observed for the photoelectric effect: electrons are immediately released, and the energy of the electrons depends on the wavelength and not on the intensity.

Hartmut Braun
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  • Can we say that the electron is ejected because of the relativistic mass and energy of the photon? If yes then what is relativistic mass of the photon doing? – Mad Dawg Jan 23 '19 at 12:26
  • Photons carry energy and momentum. There is no concept of them having mass. Usually, the electron is said to “absorb” the photon (and thus its energy). Frankly, I don’t know If there exists a model of how exactly this absorption takes place. – Hartmut Braun Jan 23 '19 at 12:37
  • How can they have momentum with no mass? – Mad Dawg Jan 23 '19 at 12:41
  • The same way they carry energy without having mass. The energy is Planck constant times frequency. Momentum is Planck constant times frequency divided by speed of light (sorry, I don’t know how to use formulars in comments). Maybe there is a concept for photon mass via the relativistic energy-mass-relation. But I haven’t seen it being used. – Hartmut Braun Jan 23 '19 at 12:51
  • @MadDawg https://physics.stackexchange.com/questions/2229/if-photons-have-no-mass-how-can-they-have-momentum – playdis Jan 23 '19 at 14:02
  • Thanks for the link. If you read the comments, however, it is again stated that thinking of photons having a "relativistic mass" may lead to confusion and is discussed quite controversial. – Hartmut Braun Jan 23 '19 at 17:45
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My question is that isn't wave capable of doing the same?

Sure it is, but the details look different.

  • For a wave, any frequency should be able to produce the effect. In experiments there is a threshold frequency and waves below this frequency will produce no photoelectrons at all. This cannot be explained in the wave picture.
  • For a wave, increasing the amplitude would increase the energy that the electrons come out with. In experiments, increasing the amplitude (really, the intensity) increases the number of electrons that come out, but it doesn't change the energy that they come out with. This cannot be explained in the wave picture.
Emilio Pisanty
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  • Okay, so its like that since wave give energy continuously so it should be able to eject electrons at any frequency but in the experiments it was found out that at certain frequency only the electrons can be ejected, right? – Mad Dawg Jan 23 '19 at 14:10
  • Now the second point tells us that on increasing the amplitude it was thought the electron should come out with more energies but they did not in experiments rather more electrons came out... So in the reality it was thought that more the amplitude, more the photons and only one photon can be consumed by an electron and when its removed from its place the electron which has taken its place will be removed after it... Right? Pls tell have i understood it right? – Mad Dawg Jan 23 '19 at 14:13
  • Yeah, that's basically it. – Emilio Pisanty Jan 23 '19 at 15:26