I Photon & Electron: Photoelectric & Compton Effects

[Sandeep T S]

How photon transfer energy to electron in case of photoelectric effect,and compton effect. Is any high level theory which explains this scenario?

 

[ZapperZ]

How photon transfer energy to electron in case of photoelectric effect,and compton effect. Is any high level theory which explains this scenario?

Compton effect: this is basically conservation of energy and momentum in a "collision", something you should be familiar with in basic kinematics.

Photoelectric effect: this requires knowledge of energy bands in a solid, because this is where the standard photoelectric effect occurs. A solid has the same idea of energy levels (in this case, continuous bands) as what you may already know in atoms. When a photon with sufficiently high enough energy excite an electron, it has the probability to escape the solid.

If you need more than this, i.e. "high level theory", then you also need "high level physics education".

Zz.

 

[Sandeep T S]

Compton effect: this is basically conservation of energy and momentum in a "collision", something you should be familiar with in basic kinematics.

Photoelectric effect: this requires knowledge of energy bands in a solid, because this is where the standard photoelectric effect occurs. A solid has the same idea of energy levels (in this case, continuous bands) as what you may already know in atoms. When a photon with sufficiently high enough energy excite an electron, it has the probability to escape the solid.

If you need more than this, i.e. "high level theory", then you also need "high level physics education".

Zz.

What are they?

 

[ZapperZ]

What are they?

They are those.

Zz.

 

[Sandeep T S]

They are those.

Zz.

I only mean that those classical term like momentum,and quanta. are they are most experimental evidant theory. Is anyone proposed any new theory which explains those phenomenas

 

[ZapperZ]

I only mean that those classical term like momentum,and quanta. are they are most experimental evidant theory. Is anyone proposed any new theory which explains those phenomenas

No.

And this is now a different question entirely, because it appears as if you're not satisfied with the answers that you were given. Why do you think the current explanation using QM is not sufficient? Do you understand the standard QM description of these phenomena in the first place?

Zz.

 

[berkeman]

Is anyone proposed any new theory which explains those phenomenas

Which part of the current explanation of the Photoelectric Effect do you think needs fixing?

https://en.wikipedia.org/wiki/Photoelectric_effect

 

[ZapperZ]

Which part of the current explanation of the Photoelectric Effect do you think needs fixing?

https://en.wikipedia.org/wiki/Photoelectric_effect

View attachment 222115

Just a caution, I have always had issues with Wikipedia entry on this phenomenon, which is why I keep "threatening" to write my own Insight article on the Photoelectric Effect. This Wikipedia entry has quite a few quirky problems.

For example:

The direction of distribution of emitted electrons peaks in the direction of polarization (the direction of the electric field) of the incident light, if it is linearly polarized.

If it pays that much attention to the direction of polarization of light, they should also pay attention to the question on whether the solid is polycrystalline or a single crystal, because this will also dictate the direction of distribution. If the solid is polycrystaline, then it doesn't matter if the incident light is linearly polarized or not, because the direction now no longer matters. If it is linearly polarized, and the solid is a single crystal, then the orientation of the crystal symmetry with respect to the direction of polarization matters.

In the X-ray regime, the photoelectric effect in crystalline material is often decomposed into three steps:

This is misleading, because the 3-step model is applied to all photoemission processes, not just in the "x-ray regime". Furthermore, this is not properly cited. The 3-step model is popularly attributed to Spicer (C. N. Berglund and W. E. Spicer, Phys. Rev.136, A1030(1964)).

I had to stop there before I pop a blood vessel!

Zz.

 

[berkeman]

which is why I keep "threatening" to write my own Insight article on the Photoelectric Effect.

Great idea!

 

[jtbell]

Compton scattering is a standard example or exercise in quantum electrodynamics (QED).

 

[vanhees71]

Well, both the Compton and the photoelectric effect do not need the quantization of the electromagnetic field to be correctly described. It's sufficient to describe the electrons quantum mechanically to reach the level of description in the usual introductory sections of QM 1 textbooks (which almost always are inaccurate). For the photoelectric effect see my Insight article:

https://www.physicsforums.com/insights/sins-physics-didactics/

 

[Derek P]

I had to stop there before I pop a blood vessel!

Have you tried editing the Wikipedia article? Just for your health.

 

[vanhees71]

To edit a Wikipedia article may be anything, but not good for your health ;-)).

 

[Derek P]

To edit a Wikipedia article may be anything, but not good for your health ;-)).

My doctor always says if a treatment doesn't work you obviously need more of it.

 

[ZapperZ]

Have you tried editing the Wikipedia article? Just for your health.

Have you tried reading my distaste for Wikipedia? It explains why I do not "contribute" to it.

Zz.

 

[vanhees71]

Well, I'm a bit more schizophrenic about Wikipedia. On the one hand I think it's among the greatest achievements of the "internet" (which I sometimes think the physicists never should have given to the public since the publicity has partially spoiled its purpose, because there is so much irrelevant information out there that the really useful information is totally hidden, but well, when you know, where to look, it's ok again). Indeed, Wikipedia is a great substitute for encyclopediae I'm used to in my early days before there was the internet. You can quickly look something up, but as with the traditional encyclopediae it's clear that it's never the full story about a subject, and if you want to really understand something and build an educated opinion about it you have to study deeper into the original literature.

On the other hand Wikipedia has one great flaw, and that's that it is anonymous, i.e., I never know who has written an entry or parts of it etc. Sometimes I know from the author, whether I tend to take seriously what s/he is writing or rather have to be critical (e.g., reading Dirac gives me more confidence than reading Bohr or Heisenberg ;-)).

Then there is my very early and very short attempt to contribute to Wikipedia. This must have been more than about 15 years ago. It was quite in the beginning of Wikipedia, and I read out of curiosity and to get an idea how much one may trust its entries, about special relativity in the German Wikipedia. It was a desaster. I thought that must be one of the very short-lived bad physics pages that will vanish as soon as they occurred on the internet. Nevertheless, I thought I could help to make it better. That was what was said on the homepage of Wikipedia: You should contribute. So I took quite some time to write up an article about SRT and then edited the article. All this efford of about a week was online for about 15 minutes. Then I learned that Wikipedia is "democratic", i.e., that I should not think that one can easily edit an article without a lot of discussion so that democratically one can decide which version of a section in the SRT article may be used finally. I'm a very democratic person as long as politics is concerned, but science is not democratic at all. There's a very strict judge called experiment and observation to decide about whether a scientific theory is correct or not, and what was in the Wikipedia was not even explaining the theory right, let alone to give justice to Einstein's theory at all. From this experience I decided, Wikipedia editing is not for me. You simply don't decide about an entry in the natural sciences by a democratic voting about its validity or not. Of course, everybody can err on something and make wrong claims or writes something wrong even about standard textbook science, but it's not decided in a democratic voting but by evidence from the original peer-reviewed literature and well-established textbooks about the subject.

That said, I must say that today the Wikipedia article about SRT is ok (even the German version) and even better than many articles about the subject in traditional encyclopediae of the pre-internet age.

 

[PeterDonis]

I must say that today the Wikipedia article about SRT is ok

The problem with Wikipedia is not that any given article is inaccurate. The problem is that the only way to tell whether a given article is accurate is to already know about the subject of the article by other means. You cannot have any confidence that a given article, on a topic you know nothing about, is accurate, just from the fact that it's on Wikipedia.

 

[vanhees71]

Of course not. That holds for almost everything in the internet. In the case of Wikipedia it also holds for traditional encyclpediae. I was pretty surprised by a study some years ago, where Nature asked experts in the natural sciences to compare Encyclopedia Britannica with Wikipedia for accuracy of the entries, and they came to the conclusion that Wikipedia on average was more reliable than Encyclopedia Britannica.

 

[HAYAO]

Compton effect is a photon scattering phenomenon, where photon is scattered off an material now with different energy. Photoelectric effect is a photon absorption phenomenon, where electron is emitted after an absorption of a photon. So we are observing two completely different mechanism of light-matter interaction. QED do explain both of these phenomena but it not completely necessary. How "high" in the level are you talking about?

 

[jeremyfiennes]

Photons interact with electrons as if they were classical particles. Does the same apply to photons' interactions with each other?

 

[PeterDonis]

Photons interact with electrons as if they were classical particles.

They do? What is your basis for this statement?

 

[jeremyfiennes]

Is that not what Compton scattering is: collisions as if between classical particles?

 

[PeterDonis]

Is that not what Compton scattering is: collisions as if between classical particles?

No.

 

[jeremyfiennes]

Wikipedia: "Light must behave as if it consists of particles, if we are to explain low-intensity Compton scattering". Electrons are also particles, and particle-partice interaction is collision. So what then is the Compton effect?

 

[PeterDonis]

Wikipedia

Wikipedia is not a valid source. Also, "particles" is not the same as "classical particles". Please take some time to learn what the term "photon" properly refers to.

 

[jeremyfiennes]

What is a reliable place to get this information?

 

[PeterDonis]

What is a reliable place to get this information?

A good textbook on quantum field theory. Unfortunately, that's a fairly advanced subject so you will need considerable background to work through such a textbook.

The following summary of a long discussion from one of the Usenet physics newsgroups provides a somewhat less advanced overview:

http://www.math.ucr.edu/home/baez/photon/schmoton.htm

A key point to keep in mind is that any massless object (like a photon--anything that always moves at the speed of light) cannot be localized the way a massive particle (e.g., an electron) can. The technical way of saying this in QFT is that Newton-Wigner localization does not work for massless fields. This is a major reason why photons cannot usefully be thought of as "classical particles moving at the speed of light" even in contexts where massive particles like electrons can (to a reasonable approximation) be thought of as classical particles.

 

[PeterDonis]

what then is the Compton effect?

It's a quantum scattering process. The key point about it when it was discovered was that it showed that light could change its frequency when interacting with electrons, and it wasn't possible to come up with a good explanation of that using the classical wave theory of light.

 

[HAYAO]

I suspect that jeremyfiennes does not have basic understanding of "interaction". There are several types of interaction. Compton scattering is only one of these types of interaction between photon and electron. In a very rough description, when the incident photon "hits" an electron, the electron "recoils" off. Conservation of momentum and conservation of energy means that the photon will now have different frequency.

 

[PeterDonis]

In a very rough description, when the incident photon "hits" an electron, the electron "recoils" off.

But that's the problem: this "rough" description is too rough, because it leaves out all the quantum mechanics. Even at a heuristic level, the interaction is not properly described as one photon and one electron "colliding", because, in perturbative QFT terms:

(a) The lowest level Feynman diagram for this process has two vertexes, not one (since each vertex connects only three lines, the incoming/outgoing electron lines and the photon line, so to get a full diagram with a photon line coming in and a photon line going out, you need two vertexes);

(b) Making correct predictions about the actual experimental data requires more than just the lowest level Feynman diagram.

And, of course, Feynman diagrams are not really direct descriptions of processes happening in spacetime anyway. (For one thing, they're usually analyzed in momentum space.)

 

[HAYAO]

But that's the problem: this "rough" description is too rough, because it leaves out all the quantum mechanics. Even at a heuristic level, the interaction is not properly described as one photon and one electron "colliding", because, in perturbative QFT terms:

(a) The lowest level Feynman diagram for this process has two vertexes, not one (since each vertex connects only three lines, the incoming/outgoing electron lines and the photon line, so to get a full diagram with a photon line coming in and a photon line going out, you need two vertexes);

(b) Making correct predictions about the actual experimental data requires more than just the lowest level Feynman diagram.

And, of course, Feynman diagrams are not really direct descriptions of processes happening in spacetime anyway. (For one thing, they're usually analyzed in momentum space.)

No doubt about it. Not that I know QFT like physicists, but there is no good classical analog to this interaction.

However, I believe that jeremyfiennes thinks there is only one type of fundamental interaction, which we need to clarify that it's not.