# Fundamental process of a photon elastically scattering from an electon

1. Oct 2, 2006

### actionintegral

I learned a lot from the discussion of the converse problem, photon absorption. What about this case? Perfectly elastic.

2. Oct 2, 2006

### inha

Is there something specific that you have in mind?

3. Oct 2, 2006

### actionintegral

Yes, the thread I refer to (inelastic scattering) generated three pages of discussion. Is elastic scattering so well understood that it prompts no discussion?

4. Oct 2, 2006

### Meir Achuz

Elastic scattering of a photon by an electron is called Compton scattering.
It is well understood in QED, but that won't stop discussion in this forum.

5. Oct 2, 2006

### inha

Compton scattering is inelastic.

6. Oct 2, 2006

### Staff: Mentor

How so? The total kinetic energy before equals the total kinetic energy after the scattering. (A photon's total energy equals its kinetic energy because it has no rest energy.)

7. Oct 3, 2006

### inha

There's momentum transfer in Compton scattering. The total momentum and energy are of course conserved but Compton scattering spectroscopy for example is always referred to as a type of inelastic scattering techinque.

8. Oct 3, 2006

### Staff: Mentor

Most any collision, elastic or inelastic, has momentum transfer. If there's no momentum transfer, the incoming particles each maintain their original momenta, which is the same result as no collision at all!

Or are you thinking of something different than I am, by "momentum transfer"? Conservation of momentum says

$${\vec p}_1 + {\vec p}_2 = {\vec p}_1 \prime + {\vec p}_2 \prime$$

$$-({\vec p}_1 \prime - {\vec p}_1) = {\vec p}_2 \prime - {\vec p}_2$$

To me, either side of the second equation gives the momentum transfer, depending on which particle you think of the momentum as "coming from."

9. Oct 3, 2006

### inha

Yeah it seems like we're using different terminology maybe due to coming from different backgrounds. By momentum transfer I mean that the outgoing photon has different magnitude of momentum than the incoming one had. That's what differentiates between inelastic and elastic light scattering techniques in condensed matter research.

10. Oct 3, 2006

### actionintegral

11. Oct 3, 2006

### RandallB

This sounds like “elastic light scattering” means no scattering of the light at all; as it just continues as before unaffected by the electron “absorbing and re-emitting” it.

Is this the type of “Absorption and reemission” that is used to account for light going though water as an example completely unchanged accept by speed while in the water?
The speed change of course accounting for refraction of light at interfaces of media imposing different speeds on the photon.

It has always seemed to me this “absorb & reemit” explanation was a too convenient, and incomplete, explanation of how the speed of light slows down in a media like water.

Is there a detailed understanding of such elastic “Absorption and reemission” effects on a photon; and what is happening to the electron as it does this?
Since the change to the photon is not completely zero as it is “slowed down” while in the media; is there any affect on the water or the electrons there in any measurable way?

12. Oct 3, 2006

### actionintegral

So Condensed Matter people refer to elastic scattering as emission and reabsorption? And they refer to inelastic scattering as a loss of energy by the photon?

13. Oct 3, 2006

### RandallB

I don't see any post here that says either of those things.

14. Oct 3, 2006

### Meir Achuz

Compton scattering is elastic. If initially at rest, the electron recoils so that the final photon has less energy than the incident photon (by the amount of energy given to the electron. In physics, this is elastic scattering. To the extent that optics is not physics, it could be called inelastic.

15. Oct 4, 2006

### actionintegral

That's what I took away from this statement

16. Oct 4, 2006

### inha

Well then there's a lot of really bad terminology used in peer reviewed journals. That's how people who scatter x-rays off stuff just talk about this. You are correct about this but I still wouldn't call RIXS an elastic scattering method although total energy and momentum are of course conserved.

17. Oct 4, 2006

### RandallB

The only way the final photon could have less energy is if it changed Frequency (Wavelength). An interaction where the light changed wavelength such as one photon being absorbed pushing an electron high energy “orbit” then emitting two photons by dropping to an intermediate energy level and then back to the original would not be considered “scattering”.

You say Compton is elastic inha calls it inelastic?
Seems to me inelastic is the better description.

18. Oct 4, 2006

### actionintegral

How so? Energy is conserved. It seems inelastic would be when the photon "merges" with the electron as in pushing it to a higher energy level.
Of course energy is still conserved in this case, but not in terms of the participants of the collision