# Photon hitting an electon

1. Mar 27, 2009

### jobyts

From the FAQ section of "why speed of photon changes in different medium".
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Moral of the story: the properties of a solid that we are familiar with have more to do with the "collective" behavior of a large number of atoms interacting with each other. In most cases, these do not reflect the properties of the individual, isolated atoms.
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If a photon can be emitted by an electron, an electron can also be hit by a photon. What happens if a photon hits an electron? Why do we have a double standard to explain the photon emission (we explain with just one atom with one electron), while to explain photon reflection we need the lattice?

Another one:
If a photon is originated from an electron, it has to start from speed zero. So, before it attains the speed c, there should be intermediate speeds, right?

2. Mar 27, 2009

### ZapperZ

Staff Emeritus
We don't have such "double standards"! You are forgetting that it depends on the situation. A photon can be absorbed by an atom (i.e. making an electronic transition). A photon can ALSO be emitted by an atom, making the SAME type of electronic transition. Where's the double standard there?

A photon can be absorbed by a METAL (i.e. a solid which has collective effect) and produce an electron. You can ALSO get an inverse photoemission, whereby you shoot electrons at a metal, and the transition can produce light! Again, no "double standard here".

You are taking only specific EXAMPLES in some cases, and then, without realizing other possibilities, somehow thinks that this is the ONLY possible scenario.

Zz.

3. Mar 27, 2009

### jobyts

Let's say one photon goes through one hydrogen atom. Will the speed of the photon decrease? Can you explain what happens in each of the following cases:
1. the photon hit the electron.
2. the photon hit the nucleus.

4. Mar 27, 2009

### alxm

Why is it the job of other people to explain this to you? Get a textbook in quantum optics.

5. Mar 27, 2009

### jobyts

Feel free not to explain. I won't be offended... but there are quite a bunch a people in this forum who are very helpful.

6. Mar 27, 2009

### BAnders1

Richard Feynman has a book called QED that explains a lot of these questions. If you go to half.com or Amazon you can find it between $5-$15. It's pretty helpful even if you don't have any background in quantum mechanics (he actually wrote it for non-scientists).

If you are looking for a more detailed, quantitative explanation, then I'd have to suggest a textbook. If no one is going to help you, help yourself. I wish I could help more.

7. Mar 27, 2009

### ZapperZ

Staff Emeritus
You will note that in the FAQ, it says that the speed of a photon doesn't change, no matter where it is traveling through. The apparent slowdown of light speed is the measured group velocity of light.

The question on what happens when a photon hit an electron is completely different question and has nothing to do with the speed of light. Your question is vague because you did not describe the scenario of the electron, i.e. is this a free electron? is this an electron bound in an atom? Is this an electron inside a solid? Each one of these produces different types of interactions and different results!

A photon hitting a nucleus, if it is energetic enough (i.e. gamma rays) can cause a number of results, depending on what nucleus is involved. There are nuclear energy states very much like atomic energy states.

You need to clarify your question because there are a large number of scenarios involved with different outcomes. Also note that this is very different than the FAQ that you cited has having "double standards". That FAQ dealt with solids, not individual atoms or individual nucleus. That's why there's a collective behavior involved. This needs to be clarified so that there's no confusion because you seem to be focusing on a completely different scenario than what the FAQ is describing. So do not mix those two.

Zz.

8. Mar 27, 2009

### jobyts

Thanks for your replies. I think I used an incorrect term "double standard" here. What I meant to say was kind of "why we don't have a unified theory?" to explain everything. As you said, I think I was trying to apply the same theory to single atom and was confused.

To the physics part,
I would like to know the simplest case. One photon (in the visible light frequency) hits a free electron. What happens to the photon and electron after the collision?

9. Mar 28, 2009

### ZapperZ

Staff Emeritus
In a sense, we do! This is all QFT/QED. They are just applied in different scenarios.

Look up, for example, Compton scattering.

Zz.

10. Mar 28, 2009

### Staff: Mentor

No. This is classical reasoning which is not very helpful for the quantum world. Also, even classically a massless particle would not need any time to accelerate.