I have a question regarding photoelectric effect. My textbook says that when photoelectric effect experiment was first performed, physicists could not explain the outcome with classical physics (e.g. maximum kinetic energy depends on frequency of light, not intensity). My question is, wasn't there even one thing that could be explained with classical physics about the results of this experiment? If anyone knows there was, please explain it to me.
Thank you in advance.
mgb_phys
Sep27-08, 08:24 PM
Classically you can explain light knocking out an electron,what you can't explain is how you need just one photon of high enough energy to do the job when billions of slightly lower energy put together can't.
atyy
Sep27-08, 08:25 PM
The original explanation was that light consisted of discrete particles called photons.
This turns out not to be necessary, and it is possible that it is just the energy levels in the atoms that are discrete.
So classical physics cannot explain the experiment, but you can choose to put the non-classicality in the atoms or the light.
However, the photon concept turns out to be necessary for many other phenomena, so it is still useful to regard photons as a correct explanation of the photoelectric effect.
musashi1029
Sep27-08, 08:48 PM
Classically you can explain light knocking out an electron,what you can't explain is how you need just one photon of high enough energy to do the job when billions of slightly lower energy put together can't.
I see. So only the thing they could explained with classical physics was that light could knock out electrons out of matter under some circumstances. Thank you very much for the very concise answer!
The original explanation was that light consisted of discrete particles called photons.
This turns out not to be necessary, and it is possible that it is just the energy levels in the atoms that are discrete.
So classical physics cannot explain the experiment, but you can choose to put the non-classicality in the atoms or the light.
However, the photon concept turns out to be necessary for many other phenomena, so it is still useful to regard photons as a correct explanation of the photoelectric effect.
I see, so without photon theory explanation is very tough. Thank you!
musashi1029
Sep27-08, 09:19 PM
Classically you can explain light knocking out an electron,what you can't explain is how you need just one photon of high enough energy to do the job when billions of slightly lower energy put together can't.
Actually, would you mind explaining to me why "billions of slightly lower enegergy put together" cannot cause electrons to leave matter?
ZapperZ
Sep27-08, 09:34 PM
Hello.
I have a question regarding photoelectric effect. My textbook says that when photoelectric effect experiment was first performed, physicists could not explain the outcome with classical physics (e.g. maximum kinetic energy depends on frequency of light, not intensity). My question is, wasn't there even one thing that could be explained with classical physics about the results of this experiment? If anyone knows there was, please explain it to me.
Thank you in advance.
You may want to read the intro in J.J. Thorn et al. paper and references therein. You can get a copy of it here:
The standard photoelectric effect strongly points in favor of the photon picture, but cannot rule out light being a classical field. It is other phenomena (photon antibunching, multiphoton photoemission, which-way experiments, etc.) that would make the convincing evidence for the photon picture.
I'm a bit hesitant to point this out since it may complicate things beyond what you've asked for and what you wish to know, but what the hey... The Einstein photoelectric effect, at least the naive version of it, can actually be violated (http://physicsandphysicists.blogspot.com/2007/11/violating-einsteins-photoelectric.html), even when using the photon picture. I'm mentioning this because of the point being made that "... billions of slightly lower energy put together..." cannot produce a photoelectron. This is only true for the single-photon photoemission, which is what the standard photoelectric effect is. We have plenty of observations of multiphoton photoemission using photons with energy LESS than the work function.
Zz.
atyy
Sep27-08, 09:48 PM
I'm a bit hesitant to point this out since they may complicate things beyond what you've asked for and what you wish to know, but what the hey... The Einstein photoelectric effect, at least the naive version of it, can actually be violated (http://physicsandphysicists.blogspot.com/2007/11/violating-einsteins-photoelectric.html), even when using the photon picture.
I remember asking my high school teacher about 20 years ago about two-photon effects in the photoelectric effect, and was told that if the intensity is high enough you see a second peak, exactly as expected (she didn't tell me about selection rules, but still a pretty good answer I think). More recently I read that Einstein himself mentioned two-photon effects in his paper. I haven't read the paper myself, and I can't quite remember what the source is ...
musashi1029
Sep27-08, 09:49 PM
>ZapperZ, atyy
I see. So if the light intensity is high enough, an electron can obtain enough energy from multiple photons with energy lower than work function before the energy decays. It is very clear now, thank you so much!
mgb_phys
Sep27-08, 10:36 PM
Not an expert but I thought you only have a 2photon photo electric effect in rather non-classical setups like nano-particles a surface plasmon or some metastable state.
I don't know that you can hit a normal electron with two photons and eject it.
Redbelly98
Sep27-08, 11:07 PM
It can also happen with more than 2 photons, at least in the case of ionizing atoms.