Photon question (collision or no collision)

1. Jan 23, 2016

Psinter

Probably a simple question, but I'm honestly at a loss here. It is well known from the photoelectric effect that an electron will accept only a specific amount of energy (quantified energy) from a photon, otherwise it will not accept it.

What happens with the photon that collides with an electron and is not accepted by said electron? Does it keeps traveling until it hits something that accepts it?

2. Jan 23, 2016

Staff: Mentor

This is not what the photoelectric effect shows. The effect shows that light transfers its energy in specific amounts of energy that depend on the light, not on the electrons.

It is true that electrons in material are not able to absorb any light at low frequencies, but once the photon has enough energy to free the electron, every photon energy works. There are not only steps.
There is not even a collision in the first place (particles are not billard balls). Yes, the light just goes on. Window glass and visible light is a nice example showing this.

3. Jan 23, 2016

Psinter

I'm sorry, but I don't get it. If it's not too complex, would you be so kind to expand a little on it? Please? Thanks. The part of the "once the photon has enough energy to free the electron" in your statement is what is baffling me since I thought that it must be an exact amount of energy. Not more not less. So if the photon has enough and more energy, will it free the electron?
I have a question on this, but I'm afraid it will be too stupid.

4. Jan 23, 2016

Mentz114

The photon is not travelling, it just comes into being in order to be accepted by the electron. So if the field has a mode (frequency) $\omega$ that matches the electrons requirement, there is a probability of interchange. Otherwise nothing happens. Photons are not little balls moving about.

The photo-electric effect is explained here https://en.wikipedia.org/wiki/Photoelectric_effect

5. Jan 23, 2016

Staff: Mentor

Yes.

You can have transitions that need a fixed energy, e. g. to change the energy level of an electron in a molecule. All those energy levels are fixed, so the photon energy has to match the transition energy. But you can also free the electron, if the photon has sufficient energy. Then the energy of the electron afterwards is not fixed, which means the photon energy does not have to have a specific value either.
There is a third option that happens in metals and other solids, where the electrons have "bands" of allowed energy.

6. Jan 23, 2016

Roger Dodger

In simple terms, promoting an electron from one bound level to another bound level is not the same as ejecting the electron. When promoting an electron from one bound level to another, the energy of the photon must match the difference in energy levels of the two orbitals. When ejecting an electron, the free electron can accept any excess energy it wants because a free electron's energy levels are not quantized.

7. Jan 23, 2016

Psinter

Thanks for your answer. You put it with words that I get.

Also, thanks mfb and Mentz114.

But here it says a photon acts like a particle. Plus he said:
Or was that a small mistake when trying to explain it? Or is he just dumbing it down so people like me can understand and photons cannot actually hit anything?

8. Jan 23, 2016

Staff: Mentor

The trick here is that the word "particle", as used in quantum field theory (including quantum electrodynamics, the theory that describes photons and electromagnetic interactions) means something very different than it does in informal English. Thus, a photon is a (quantum) particle, but it is not like a little billiard ball, it doesn't have a position except at the moment that it interacts with matter, it doesn't move through space like a bullet, it doesn't hit things, a beam of light is not photons flowing by the way a river is water molecules flowing by.

This unfortunate situation is a historical accident. When we first encountered quantum phenomena more than a century ago, all we had to work with was classical physics, and in classical physics everything behaves like little billiard balls (or groups of them). So it was natural to assume that the photoelectric effects was caused by something like a little billiard ball hitting the electron, and it was natural to call that something a "particle", just like the electron. By the time we realized that neither the electron not the photon behaved anything like little billiard balls it was too late - we had started calling them particles, and the name stuck.

9. Jan 23, 2016

Psinter

Ohhhhhh. I cannot express how taken I am by the high-quality of answers here at PF.
This leaves me at a loss for words. Specially the "it doesn't have a position except at the moment that it interacts with matter" I want to know more about photons. I'll try to get a book. Thanks again everyone.

10. Jan 23, 2016

Staff: Mentor

One of the better books you could try is Feynmann's "QED: The strange theory of light and matter", which does an excellent math-free layman-friendly explanation.

11. Jan 23, 2016

Psinter

Thanks! Had I found nothing to my level I was planning on asking here at PF for recommendations. Hihi. That one it is then.