# Light is absorbed by an atom

1. Apr 20, 2005

### jet10

As far as I know, when light is absorbed by an atom, it is emitted again in an arbitrary direction.
1) what happens to the momentum?
2) why does light always reflect in a certain direction, depending on its incident direction, if reflection is an effect of emission after absorption?

Thanks

2. Apr 20, 2005

### ZapperZ

Staff Emeritus
Notice that there is a HUGE atom in the vacinity. This beast is the one doing the recoil, so you hardly notice it.

Now this is a slightly different situation than above, because you will notice that reflection occurs not from an atom (or even a gas), but rather from solid surfaces like metals. This should tell you that there is a different mechanism for reflection here. The most common and dominant process is due to the presence of conduction electrons in metals (it is why metals make very good mirrors). To know why light reflect at a certain direction would require knowledge of the conduction band dispersion. However, it is sufficient to note that the PARALLEL momentum of the original light is conserved (technical explanation: conduction electron can make a k --> k+G transition, where G is the reciprocal lattice vector for a Bloch crystal), while the rest of the solid recoils and retransmit, in the opposite direction, the perpendicular momentum. Thus, when you have the same parallel momentum, but an equal but opposite perpendicular momentum, you get the familar reflection law.

Note that there is a 180 degree phase change of the reflected light, because the conduction electrons react in the "opposite manner" to the incoming light, and thus they reradiate light with an opposite phase. If we have positrons making up the conduction band, we won't have such a phase change.

Zz.

3. Apr 20, 2005

### jet10

Though I learned it recently, I am still not very familiar with the term reciprocal lattice. As far as I understand. This relation k-k'=G has to hold in order to observe a maximum intensity. How does this affect the conservation of parallel momentum or the recoiling of the perpendicular?

Electrons in metal can jump into the conduction band very easily. This makes it easy for them to absorb and reemit. But how does this help us with the direction of reflection? I haven't heard about conduction band dispersion before. I think I should read about it. But would be glad if you could explain to me a little.

How is it that only the perpendicular momentum recoils but not the parallel? It reminds me of tennis. The ball hits the ground and bouces back, but it keeps going the same horizontal direction. What happens there microscopically?

4. Apr 20, 2005

### ZapperZ

Staff Emeritus
I have no clue on how to explain this to you without invoking the recriprocal lattice, the first Brillouin zone, and extended Brillouin zone scheme, etc. Since you haven't gotten to this yet, I think all my explanation will just simply won't make any sense right now.

Also, note that electrons do NOT need to jump into the conduction band in a metal. They are ALREADY there, even at T=0K. The conduction band in a metal is occupied already. It is in an insulator and semiconductor that the conduction band is empty at T=0K.

Zz.

5. Apr 20, 2005

### Claude Bile

Reflection is not the same process as absorption and reemission as I understand it. If the reflecting surface did absorb the photon, the surface would be black, and you would not see any reflection at all.

Reflection is actually a photon driving an electron (or electrons), and the electrons retransmitting the photon (much like an antenna).

I thought this might be where some of the confusion is coming from.

Claude.

6. Apr 21, 2005

### jet10

Claude, looking at it from the classical ED point of view, the electric field forces the electrons to vibrate and thus at the same time, the electrons regenerate the photon again. Is this what you mean?

7. Apr 21, 2005

### Gurde

I have just read a book on this subject: QED: the strange theory of light and matter by RP Feynman. It is quite easy to understand! The fact is that i can't give a conclusion in a few lines...

Greetz :tongue:

8. Apr 21, 2005

### Claude Bile

Yes.

Claude.

9. Apr 22, 2005

### jet10

My problem is to think in qm terms. I think I need to go study Solid State Physics first.

10. Apr 22, 2005

### juju

As I understand it from reading, a single photon has a quantum probability of reflecting in a variety of directions around the point of impact. It is only a beam of many photons that appears to obey the classical model.

juju

Last edited: Apr 22, 2005
11. Apr 25, 2005

### jet10

correct me if I am wrong, juju. Do you mean that the atoms reemits the light in an arbitrary direction, but since we are speaking about many many photons, it seems as if it is making a spherical wave, interfering with those waves reemitted by its neighbouring atoms? Only photons in a certain direction interferes constructively. One of these directions is at the reflection angle?