# Absorption of photons by electrons

1. Oct 22, 2007

### Davnewil

Hi, my name is David. I'm a High School student in Virginia. I'm doing a research project that concerns controling the frequency and wavelength or energy of a photon that is "absorbed" and emitted by an electron. Here are some assumptions and questions I am working on. Any help would be greatly appreciated:

1. If two 3.5 n photons are "absorbed" by an electron cloud, one 4n photon can be emitted.
2. Supposedly when light hits matter, it asorbs the colors you don't see visually and "reflects" the colors you do see. Is this true or is it that the electrons absorb only certain colors/wavelength/frequencies or light and then emit those same colors and THOSE are the colors you see?
3. Why does an electron only absorb certain wavelengths and photons and emit only certain wavelengths and photons (i.e. the same photon it absorbed)?
4. How is the frequency needed to excite an electron determined? In other words why does a certain atom need x amount of energy for an electron to jump to the next level and not less or greater than x?
5. When an electron cloud is polarized it resonates with the frequency of the photon that was absorbed by it. This resonation in turn interacts with the electrons electromagnetic field to emit the same frequency as was absorbed by the electron. So does the electrons electromagentic field in the end determine what frequency and thus color of light/photon is emitted from the atom?
6. If this is true, would it be possible to change, with some kind of outside, controlable force, the electron's electromagnetic field and thus the way it is polarized by light or vice versa? This could in turn control the color of light that is emitted from the electron, making it different than what was absorbed (i.e. white light).

I love science, in particular physics, but I have not had any advanced courses in it. If there is any major problems with my reasoning, please do not hesitate to tell me. I joined this forum to learn of these problems, and to see if my research project is realistic and possible. I think i understand these concepts, but there is a very good chance i'm wrong. Please post some replies to help me. Thanks.

Last edited: Oct 23, 2007
2. Oct 22, 2007

### Mentz114

Hi Davnewil, welcome to the forum.

In q1, what does 3.5n mean ? You must state your units.

Your questions are long and show some little misconceptions. Have a look here to see what makes atoms have discrete spectra,
http://hyperphysics.phy-astr.gsu.edu/hbase/hyde.html

Free electrons can emit/absorb photons see here http://en.wikipedia.org/wiki/Bremsstrahlung

The fact that atoms have discrete spectral lines started quantum theory, which best explains what's happening between light and atoms.

3. Oct 23, 2007

### Davnewil

There is an equation on the site you gave me on discrete spectra that shows the energy for an electron in orbit around a nucleus. The equation has me a little confused. It's under quantized energy states. I think it is basically describing how the energy of an electron is determined by the atomic number or energy of the nucleus. So the energy needed by a photon to excite an electron one energy level is determined by the atomic number?

4. Oct 24, 2007

### Mentz114

I presume you are referring to the Bohr orbit. The atomic number does appear in the formula, but other things also affect the spectrum.

There's no method for acurately predicting atomic energy levels. The Dirac equation gives the energy levels of Hydrogen rather well, but there are corrections for the vacuum also.

5. Oct 24, 2007

### Davnewil

Free electrons can emit/absorb photons see here http://en.wikipedia.org/wiki/Bremsstrahlung

Wait, do you mean that only free electrons absorb/emit photons, and not bound electrons?

6. Oct 24, 2007

### ZapperZ

Staff Emeritus
Bremsstralung is not exactly free electrons emitting photons. When electrons are being slowed down by matter, such interactions no longer make them "free". This coupling is required as part of its "braking" that resulted in the emission of photons.

Look at it this way. Take a free electron that is free of coupling to anything else, and make it "absorb" a photon. Now look at what conservation laws are violated there.

Zz.

7. Oct 24, 2007

### Davnewil

Also is the energy of an electron a constant? For example the electron does not make the element, and electrons are exchanged and shared by different atoms all the time in chemical bonding. However if they are constant, why are some electrons placed in higher energy levels and some in lower energy levels? Is it because each electron has different spin-orbit interactions or electromagnetic fields?

8. Oct 24, 2007

### Davnewil

Sorry, i guess i have a lot of questions. I really havn't had any physics courses, so there are probably some major holes in my reasoning. I guess my most important question is: what controls the electron's electromagnetic field?
Does it have to do with the spin-orbit interaction? Does the atomic number determine the electron's energy level and thus its E-field? I don't know.
I do know that when a photon interacts with an electron, it makes the electron cloud oscilate at the same frequency as the photon. This displaces the electron cloud and causes the whole atom to oscilate at the same frequency. The oscilation continues through the electron cloud and atom in the same direction due to conservation of momentum. When it reaches the other end, the "oscilation" interacts with the electromagnetic field of the electron and emits a new photon with the same frequency as the orginal one.
So in some ways it could be said that the E-field of the electron controls the photon that is emitted. This is why i want to know what make up an electron's E-field. My research project is centered around this.

9. Oct 24, 2007

### ZapperZ

Staff Emeritus
Er.. you have a rather severe misunderstanding of atomic physics.

Unfortunately, what you need is a course in intro to modern physics to even begin address all of those questions. It is impossible to do what is typically done in a class over a period of a semester on a public forum. Ask your instructor for proper sources relevant to what you wish to do.

Zz.

10. Oct 24, 2007

### Davnewil

This raises another question though. If the photons all just "oscilate" through the electrons, being emitted in the same direction, why does most matter appear opaque? For example in glass, this explanation makes sense. The photons are absorbed by the electrons, oscilate through in the direction they came from, and are emitted again on the other side through the E-field interaction in the same direction. Thus light passes through glass.
For matter like copper however, this doesn't make sense, because obviously light does not pass through the same way. With an opaque material, are the photons absorbed by the electrons, and then emitted again back towards the source of the photons? So we see the color of the matter as the frequency of the photon that is emitted or "reflected" back towards us?

11. Oct 24, 2007

### Davnewil

Like i said, lots of questions. :)

12. Oct 24, 2007

### Davnewil

So, basically everything i was talking about just now is completely wrong? well i knew that was a possibility. That's why i was asking. I'm going to talk to the Physics teacher at my high school, but I'm in tenth grade right now, so my resourses are slightly limited in terms of taking a "physics course". I'm not asking you to teach me the whole course. That's maybe just a bit much. :) I mainly just want to learn something about this topic from a source that I'm sure to get good, solid information from. I know i don't have the background teaching to actually learn too much at the current time, but I have lots ideas that I just think up. While probably 9 out 10 or 99 out of 100 of those ideas will amount to nothing due to some aspect of physics I don't know, there's probably going to be the one that will be good. I just want to learn from those 99 mistakes.
so please, don't even try to teach me the course, but if you have any spare time and information, it would be welcome.

Last edited: Oct 24, 2007
13. Oct 24, 2007

### Davnewil

By the way, where do you go to school or have a teaching position?

14. Oct 25, 2007

### Mentz114

Davnewil, ask your teacher to recommend some books - you need to read lots of books. Not everything you say is wrong, but the subject is too big to be understood by listing 'facts'. There are underlying models which it takes about 6 years to grasp properly.