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Why are photo electrons emitted instantly from metal surface just nanoseconds after the light falls upon it? How does the quantum theory of radiation explain it? Why can't classical physics explain this?

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    $\begingroup$ A nanosecond is an extremely long time for dynamics that involve electrons. $\endgroup$ Jul 15, 2018 at 12:04

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The photoelectric effect, which is what you are describing, is one of the basic experimental effects that forced the invention of Quantum mechanics.( The other reasons were black body radiation and the atomic spectra.)

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Classically there should not be this behavior, because classically the frequency of the light should not play a role in the ejection of electrons, only the energy of the classical light beam was expected to affect the ejection of the electrons.

Nanoseconds are not "instantly". It is within the $Δ(t)$ allowed by the quantum mechanical solutions for the specific interaction : photon hitting atoms and ejecting electrons, releasing them from the atomic/molecular binding.

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  • $\begingroup$ Classically we should see this type of behavior if photons as particles were taking more serious. A photon of blue light has a much higher frequency than a photon of red light. A blue photon would oscillate with Kinetic energy much faster than a red photon. The oscillating energy of the blue photon would be much faster and stronger. Giving it just enough energy to knock an electron loose. The linear speed of the photon would have very very little to do with it. $\endgroup$ Jul 15, 2018 at 16:17
  • $\begingroup$ @BillAlsept the whole point is that in classical electrodynamics, maxwell's equations, light does not consist of particles, but of alternating E and B fields $\endgroup$
    – anna v
    Jul 15, 2018 at 17:09
  • $\begingroup$ That’s what I meant by “if photons as particles were taken more seriously” The way photons are considered , at least on this site,why bother even saying duality? $\endgroup$ Jul 15, 2018 at 18:33
  • $\begingroup$ @BillAlsept duality appears in the crossection/interaction behavior which shows wave characteristics. that is why we bother, because we want to understand interaction crossections $\endgroup$
    – anna v
    Jul 16, 2018 at 4:24
  • $\begingroup$ Yes I know duality is discussed but the particle aspect is never taken serious. The particle aspect is the best way to discuss or describe interactions. Light Waves cannot physically be described. $\endgroup$ Jul 16, 2018 at 4:26
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According to quantum mechanics The energy is transmitted by photon to electrons by the collision between the two. Only those photons can eject electrons which have energy more than or equal to a minimum required energy (threshold energy). Since energy is transferred in a lump, the ejection is instantaneous.

According to Classical mechanics the energy of light is distributed equally on its wavefront. So when light hits the surface, each electron gets a part of the total energy. So, the electron will acquire the sufficient energy in some time. So there must be a time lag between the striking of light and ejection of electron which is not true. Hence classical mechanics fails in this aspect.

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