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thaiqi
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photoelectric effect is now explained in quantum idea. Why the classical model fails? Are there articles computing it in detail using classical model?
It's hardly surprising that some phenomena just cannot be explained without QM - that's why we had to invent it!thaiqi said:Summary:: photoelectric effect is now explained in quantum idea, how about the classical?
Why the classical model fails?
sophiecentaur said:It's hardly surprising that some phenomena just cannot be explained without QM - that's why we had to invent it!
Einstein got his first recognition (iirc) for explaining the photoelectric effect. If you want electrons to be emitted from a metal surface then it needs to be hot enough for a significant number of them to have enough energy. Thermionic emission needs a HOT cathode. Photo emission would just not be expected to happen unless there are discrete packets of energy (i.e. photons) to target individual electrons and give them enough energy to escape from a cold surface.
thaiqi said:photoelectric effect is now explained in quantum idea. Why the classical model fails? Are there articles computing it in detail using classical model?
The insights article makes sense, I guess. I haven't read it in detail but is it not just replacing one aspect of probability with another, in order to account for one particular electron achieving the work function. Isn't it just a longer winded alternative description - in the same way that there is always an alternative way to deal with an 'obviously wave' phenomenon in terms of photons and vice versa. Diffraction is hard work to explain in terms of probability but you can get there.vanhees71 said:the photoelectric effect is perfectly understandable within the semiclassical theory (classical light shining on a quantized electron).
It doesn't really excite anyone, it's mostly interesting because of how it interacts with the history. The photoelectric effect is widely cited as proof of the quantization of light and was accepted as such at the time. BUt is it really that, or was it another example of the common experience of arriving at the right conclusion by the wrong path?sophiecentaur said:Perhaps a "semiclassical' theory doesn't grab me.
Only the winners get to write the history. We won't know who was actually right for some while. I think.Nugatory said:arriving at the right conclusion by the wrong path?
How to compute the time? It is not an possible task for me. Are there any articles on it?(I can't download Scully's article fully.)vanhees71 said:...That time for usual light sources is far longer than the about 10−9s observed in experiment,...
Alternative to what? The photoeffect can be properly understood only with quantum mechanics. The most simple explanation is the one given in the Insight article, using the semiclassical theory. On this level you don't gain very much using the full quantum field theory. Also one should keep in mind photons are always a wave phenomenon.sophiecentaur said:The insights article makes sense, I guess. I haven't read it in detail but is it not just replacing one aspect of probability with another, in order to account for one particular electron achieving the work function. Isn't it just a longer winded alternative description - in the same way that there is always an alternative way to deal with an 'obviously wave' phenomenon in terms of photons and vice versa. Diffraction is hard work to explain in terms of probability but you can get there.
Perhaps a "semiclassical' theory doesn't grab me. It's rather like being a bit pregnant.
The idea is to assume classical electrodynamics. Then take a light source which radiates light with a power ##P##. The intensity is this power per unit area. Assuming a isotropic radiation you get ##I=P/(4 \pi r^2)## for the intensity at a distance ##r## from the light source. The time it takes to accumulate the binding energy ##W## is ##\Delta t=W/(I A)##, where ##A## is the area over which the electron is distributed when bound at an atom, which can be estimated by using the Bohr radius ##r_{\text{B}}##, i.e., ##A=\pi r_{\text{B}}^2##. So finally you getthaiqi said:How to compute the time? It is not an possible task for me. Are there any articles on it?(I can't download Scully's article fully.)
Thank you very much for such a detailed reply.vanhees71 said:The idea is to assume classical electrodynamics. Then take a light source which radiates light with a power ##P##. The intensity is this power per unit area. Assuming a isotropic radiation you get ##I=P/(4 \pi r^2)## for the intensity at a distance ##r## from the light source. The time it takes to accumulate the binding energy ##W## is ##\Delta t=W/(I A)##, where ##A## is the area over which the electron is distributed when bound at an atom, which can be estimated by using the Bohr radius ##r_{\text{B}}##, i.e., ##A=\pi r_{\text{B}}^2##. So finally you get
$$\Delta t=4 \pi r^2 W/(\pi r_{\text{B}}^2 P).$$
From Halliday-Resnick I get some numbers, where this is given as an example: Consider a light source with ##P=1.5 \; \text{W}##, ##r=3.5 \; \text{m}## and ##W=2.3 \; \text{eV}## (for potassium) and ##r_{\text{B}} \simeq 5 \cdot 10^{-11} \; \text{m}##. Then you get ##\Delta t \simeq 4580 \; \text{s} \simeq 1.3 \; \text{h}##. So it would take more than an hour until you get an electron out of the metal.
One observes however that the photocurrent starts immediately, which is another hint that the photoelectric effect cannot be explained with classical electrodynamics.
Some textbooks say that in the classical model the light absorption by electron should only have something to do with the light density and have nothing to do with the light frequency. I think this is incorrect. The electron should oscillate within the light wave and will have something to do with the frequency, am I right?vanhees71 said:There's no difference. The point is that you have to treat at least the electrons relativistically to get the photoeffect described right, i.e., in accordance with observations.
I think that mechanical view is not particularly relevant. I don't think the idea of a wave 'shaking an electron loose' helps. Apart from the threshold photon energy, there is no 'resonance' involved (the dreaded Hydrogen Atom model comes to mind here and it's not appropriate at all in a solid metal). Whatever the frequency of the incoming photon, the KE of the electron will be (up to and including) the surplus energy.thaiqi said:The electron should oscillate within the light wave and will have something to do with the frequency, am I right?
sophiecentaur said:I think that mechanical view is not particularly relevant. I don't think the idea of a wave 'shaking an electron loose' helps. Apart from the threshold photon energy, there is no 'resonance' involved (the dreaded Hydrogen Atom model comes to mind here and it's not appropriate at all in a solid metal). Whatever the frequency of the incoming photon, the KE of the electron will be (up to and including) the surplus energy.
vanhees71 said:There's no difference. The point is that you have to treat at least the electrons relativistically to get the photoeffect described right, i.e., in accordance with observations.
We already have the word "photon". Why introduce the confusing term "wave train"?thaiqi said:One wave train is emitted by one electron transition, ...
I can see why it annoys you but the word photon is a bit entrenched and needs qualifying to remind people it's not a little bullet. The term 'wave train' is fine for me and it extends into concepts of coherence.A.T. said:We already have the word "photon". Why introduce the confusing term "wave train"?
The Photoelectric Effect is a phenomenon in which electrons are emitted from a material when it is exposed to light. This effect was first discovered by Heinrich Hertz in 1887 and later explained by Albert Einstein in 1905.
The Photoelectric Effect is related to Classical Computation because it was one of the key experiments that led to the development of quantum mechanics. Classical computation of the Photoelectric Effect involves using classical equations and principles to explain the behavior of electrons in the presence of light.
Some key articles on Classical Computation of the Photoelectric Effect include "The Photoelectric Effect in the Classical Theory of Radiation" by A. Einstein, "Classical Computation of the Photoelectric Effect in Metals" by E. Rutherford, and "The Photoelectric Effect and the Quantum Theory" by N. Bohr.
Classical Computation has limitations in explaining the Photoelectric Effect because it cannot fully account for all of the observed phenomena, such as the energy distribution of emitted electrons. This led to the development of quantum mechanics, which provides a more accurate explanation of the Photoelectric Effect.
The understanding of the Photoelectric Effect has evolved over time from its initial discovery and explanation by Hertz and Einstein, to further experiments and theories by scientists such as Rutherford and Bohr, to the development of quantum mechanics. This evolution has led to a deeper understanding of the nature of light and the behavior of electrons, and has paved the way for advancements in technology such as solar panels and photodetectors.