mo42

Here's something that's always bugged me. Whenever you have something like a mirror, or a shirt, or a solar cell, or whatever, and it's hit by light or other electromagnetic radiation, science has told us that the atoms absorb the light and electrons jump up an energy level and a photon gets released again when the electron jumps back down.

But how the heck does the atom pull this off? Does light hit the electron? Does it hit the nucleus? Does passing close by the atom cause the photon to be sucked in? What's the deal?

I would think the hitting the electron would be the most likely theory, but that just seems like it might be hard to do.

Anybody know how this works?

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NeoRete

I might not be able to explain this perfectly, as I took physics a year ago, but I'm going to try and take a stab at it.

Light is made of photons, packets of energy that have no mass. While conceptually photons exist as discrete packets of energy, in reality they are like any form of electromagnetic radiation which exists as waves.

Light can be absorbed, reflected, or transmitted. Absorbtion takes place when light photons hit an atom, and the photon's energy is taken in by the atom without exciting an electron. Reflection is the null opposite of this, the photon is not accepted by the atom. Transmission takes place in the way you describe it, by exciting an electron. Most atoms exist in a stable form called the ground state. In this state, an atom can only be excited by a photon of a specific wavelength (photons exist as waves). If this certain wavelength is absorbed, an electron is excited and another photon is emitted, bringing the atom back down into the ground state.

Electrons conceptually exist in electron shells around a nucleus, the distance from the center of the nucleus translates into an energy level. As a photon enters this electron cloud in absorbtion and, its energy is added to the electrons for that atom. The only way a photon can be emitted is if the photon being absorbed has a high enough energy to bump an electron up to the next energy shell.

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mo42

I understand that, except for the "hitting the atom" part. How does "hitting the atom" jump the electron up in energy? What part of the atom does it hit? As I said, I would guess electron if I had to, because it's the thing that is affected by it, I haven't read that in any physics books, just wondering if there's a source.

As for reflection, how is the photon reflected? Is there some kind of insane localized super-gravity field that the electron runs into and then bends and shoots off the other way compared to the plane of electron shells created by the mirror? Or does the photon actually "bounce" like a ball off the surface of the earth?

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MSD

A photon is a unit of energy. This energy is absorbed by an electron, which jumps up to the next highest energy state. This is an unstable state, so the electron naturally falls back down into its ground state almost immediately. When this happens, there is more energy than necessary to keep the electron in its orbit, and a photon is emitted from that electron.

namnori

Yeah, you can explain this a variety of ways. Maxwell's equations, quantum mechanics. There's a lot of underlying theory that needs to be explained first if you want to get a full appreciation of what's going on.

mo42

Do we know if it actually has to *hit* the electron or not? I would think the odds of the essentially massless photon and excessively small electron hitting each other as the photon passes through the electron cloud would be extremely small. Of course, I suppose most things that absorb light, paint chips for example, have to be several million (or some larger order of magnitude) atoms thick for even the thinnest chips.

Does anyone know if any experiments were done to test the odds of a photon being absorbed by a given atom? (presumably the experiment would be done with a photon with proper wavelength to be absorbed by the atom in question)

Or any experimental evidence to support the mechanics of how a photon is absorbed by an electron at all?

__________________
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NeoRete

When speaking about these quantum interactions, most of it is conceptual.

A massless photon doesn't "hit" an electron when it is absorbed. Because it is massless and basically made of energy, that energy is absorbed by the electron which has been discussed. There are quite large gaps between molecules, but there are electrical and weak nuclear forces which account for many of the interactions we are talking about here.

http://acept.la.asu.edu/PiN/rdg/visnxray/visnxray.shtml can explain better than I the basics of the process of photon absorbtion.

It's well understood what wavelengths excite certain types of electrons. http://wine1.sb.fsu.edu/chm1045/note...m/Struct02.htm discusses how to calculate this.

Here is a study observing multiphon absorption mechanism of TMTTF (a polarized molecule) http://www.edpsciences.org/articles/...jp4114024.html Click on the pdf link at the top for the full article

I apologize for all the links, but there is no easy way to summarize all this Quantum Theory in any concise form.

__________________
Charlie was a chemist but Charlie is nomore, what Charlie thought was H2O was H2SO4

quicksteal

Despite currently taking a class on quantum mechanics, I still had trouble answering this one. Y'all seem to be hung up on electrons absorbing energy. Entire atoms and molecules also absorb energy. The bonds between two atoms can become excited and reach a higher vibrational energy state. Also, the way that atoms rotate around each other can be changed with the input of energy (via a photon).
This gives us the ability to identify an atom by IR (infrared) spectroscopy. Remember that there are extremely few energies that can be absorbed by a given molecule, and these energies are characteristic of the type of bond in the molecule in the case of IR. This site describes IR decently: http://www.shu.ac.uk/schools/sci/che...ec/irspec1.htm
So it's not just an electron with different energy states, there are other energy states that relate to molecules and their interactions.

fckm

I think you all are concentrating too much on the particle aspect of light. Remember that the most interesting thing about quantum mechanics is duality; things can both be particles and waves. In this case, Light is simply a wave in the electromagnetic field. The field is localized to some region surrounding the atom. When the wave, which carries energy, reaches the atom, the atom can simply absorb the energy from the field. The amount of energy that it can absorb is quantized, thus the photon.

The photon is a product of the interaction of light with matter. Light is not a stream of bullets. Photons do not have to "hit" the "target". The concept of a photon only comes into existence when you talk about interactions between matter and an electromagnetic field. The field permeates the region surrounding the atom. The atom simply gives or recieves energy from the field through quantized packets called photons. Don't get too hung up on the particle nature of light, and don't forget about the wave nature of light.
Remember, nothing in quantum mechanics is completely localized. Particles are not little bb's. Everthing is a probablistic wave packet. There are no targets, and there are no bullets. Only interactions between probabilities.