# How can light interact with atoms?

1. Feb 16, 2013

### Menaus

Atoms are much, much smaller than the wavelength of visible light. Visible light is 4,000 - 7,000 angstroms in wavelength while atoms are ~1-7 angstroms.

How can something interact with a wavelength when that wavelength is 10^3 larger than it?

2. Feb 16, 2013

### Staff: Mentor

Can you swim in water waves which are longer than 2m? Do they interact with you?
You have a similar situation with visible light and atoms.

3. Feb 16, 2013

### Menaus

Water waves are different from electromagnetic waves.

If I send a EM wave with a wavelength of 5000m to a 1m antenna, there will little if any interaction.

4. Feb 16, 2013

### Staff: Mentor

Atoms are not metallic antennas. In a semi-classical picture, the electrons can follow the (slowly changing) electromagnetic field, and get in resonance with that field.

5. Feb 17, 2013

### Jano L.

Why do you think that?

6. Feb 17, 2013

### Staff: Mentor

Note that if you are referring to the fact that visible light interacts with bulk materials, you cannot think of it as interacting with single atoms at a time. The EM wave interacts with the material as a whole.

7. Feb 17, 2013

### DrDu

You must be of the new generation which hasn't even seen medium and long wave radio receivers (best with valves and ferrite antenna) any more!

8. Feb 17, 2013

### Menaus

That's a little different, those radio receivers are already powered by a source so that their electric or magnetic fields are larger, and thus, the antenna is electrically larger.

So are you saying that atoms do the same thing? That would make sense to me.

It's true? It's why many LF receivers had very large antennae in the early days, we didn't know about the aforementioned effect.

I'm referring to the fact that visible light interacts with atoms, why would I infer anything else?

They don't have to be metallic to be antennae. ;)

Unless the electron is the correct size, it shouldn't be able to get in resonance with a wave larger than itself, correct? If I hit a pendulum at a uniform rate which is lower than its resonant frequency, it cannot be in resonance. Of course, if we take the aforementioned effect then the atom could have a large electric field which then makes it *look* electrically bigger and thus, get in resonance the proper way.

Just note, I'm not here to argue, I just it all to make sense. :)

9. Feb 17, 2013

### ZapperZ

Staff Emeritus
If this is true, we will not have particle accelerator, such as the one at Fermilab and CERN. These particles are accelerated by RF signals. So either such a thing never happens, or your understanding is faulty.

Zz.

10. Feb 17, 2013

### MikeGomez

Hi Menaus,

The wavelength as emitted or absorbed has more to do with energy levels (mass and velocity of electron) than just the raw wavelength of a released photon.

λ = hc/ΔE

Do a google search for emmission spectrum and Bohr model.

Last edited: Feb 17, 2013
11. Feb 17, 2013

### Naty1

Hi Menaus...
Based on your implied model, interactions would not be well defined....that's not the model of quantum theory.

Note that when you shine a flashlight on a piece of wood for example, not much happens. More happens if the material happens to be photoelectric. And of course a LASER can set a piece of wood on fire, so there is definitely something of interest here between light and matter.
One way to gain insights about wave-matter interactions is to read about the 'photoelectric effect'....you'll find its the energy of individual photons that controls the interaction...a valuable hint from Einstein!!

Sub atomic particle interactions [even with 'continuous' electromagnetic waves] are best thought of as interactions of field [wave] quanta, that is photons. Quanta are localized 'particle' aspects of waves...localized energy packets. In other words, the Standard Model of particle physics, which describes particles and their interactions, is based on quantum theory. So if you check here:

http://en.wikipedia.org/wiki/Standard_model_of_particle_physics#Particle_content

you'll see everything is described in terms of particles....light is represented as photons for example.

In quantum theory, particles are are described via the continuous wave function in a higher-dimensional space in which the wave function exists, what physicists refer to as ‘configuration space’. So while the evolution in time appears continuous and deterministic, photons, like all quantum particles, interact at a single point statistically, regardless of the size of their wave packet dimension. Quantum activity is largely a statistical phenomenon so it doesn't comport in general with everyday macroscopic observations and classical theory....in which everyday things SEEM continuous, like time and distance and lightwaves.

To learn more about the wave=particle duality of light, check out 'double slit experiment'. Richard Feynman says if you understand that experiment you understand quantum mechanics...

A related way to think about light and matter particle interactions is to note that both exhibit wave particle duality; in other words, even matter particles have a characteristic wave, a DeBroglie wave....reflecting the wave-particle (dual) nature of matter. So in classical theory there is nothing but a point particle, quantum theory unfolds a more complete picture revealing wave-particle duality, Heisenberg uncertainty,etc,etc.

Last edited: Feb 17, 2013
12. Feb 17, 2013

### Jano L.

OK, I understand your point. The answer is, as mfb said, that the wave-mechanical model of the atom is quite different from the model of, say, simple center-fed half-wave dipole antenna.

Resonance in such antenna occurs when the length of the antenna satisfies

$$d = m\frac{\lambda}{2},~~, m= 1,2,3,... (*)$$

so indeed such antenna has to be as long as the wave it is to be in resonance with.

The atom, however, is modeled differently. It has resonance frequencies too, but these are not given by such formula. That formula is not something fundamental - there are even macroscopic antennae that do not obey it either.

For example, you can receive long wave transmission with pocket radio. Antenna in such device has additional network attached to it, which influences the resulting resonance frequency of the antenna. Changing parameters of the network allows you to change this resonance frequency, so you can be in resonance with any transmission you want, within some range of course.

For the atom, things are more complicated and the standard understanding of its resonances is based onm Schroedinger's equations rather than on the antenna theory (which is based in the macroscopic electromagnetic theory anyway).

Physically, the differences are these: in the simple antenna, the resonance frequency is determined mainly by the simple boundary conditions for the current in a straight wire, since in metal there are no effective forces on the charge.

In the atom, however, besides boundary conditions (which are very different by the way) there are long-range binding forces due to nucleus as well as repulsing forces between the electrons. These co-determine the resulting resonance frequencies of the atom, and it turns out that resonance light has two orders longer wavelength than the size of the atoms.

The resonance frequencies of the atom can be determined by the time dependent Schroedinger's equation, which leads to formula for the resonance wavelength of the atom

$$\lambda = \frac{hc}{E_n -E_m},~~~n, \neq m = 1,2,3,...$$

where $E_n,E_m$ are solutions of the time-independent Schroedinger equation

$$\hat H \Phi_n = E_n\Phi_n.$$

To summarize, the atom is a microscopic object, modelled differently than simple antenna and that allows us to explain its different behaviour.

13. Feb 17, 2013

### davenn

in this universe they do. I have never seen a non metallic antenna in all my years of dealing with transmitters and receivers.

Dave

14. Feb 17, 2013

### Menaus

Uh... Trees (used as VLF/ELF receivers to study natural Earth EM waves)? Pretty much anything conductive can be used as an antenna. I could use my own body as an antenna, although I would probably die in the process, along with an assortment of other engineering problems.

The electric nature of the electron and proton would definitely suffice as an antenna. In a way, the atom could be looked at as a LC circuit seeing as how the electron and proton are connected electrically (forming the 'capacitor'), and the electron itself is spinning (forming the 'inductor')

Oh yes, I understand particle-wave duality and the like.... I'm just used to thinking of light in the way of waves (I am studying to become an electrical engineer eventually).

Thank you, this helps. :)

I'm not saying that it can't happen at all, or doesn't happen. I'm saying under the logic presented it can't happen, or we would have to throw out everything we know about resonance (Or, I suppose, say that atoms are different. ;)), which we know is fact.

I was showing that the logic doesn't follow, so it doesn't then make any sense to me. You don't seem to be following what I've been saying otherwise you would understand that I didn't infer that the quoted sentence was true.

Why are you reverting to an argumentative tone when I am trying to get the answer to a question that in a way that makes sense to me? Look forward two sentences and you have my actual postulate.

Thank you, this answers my question much better.

15. Feb 17, 2013

### chill_factor

how does a IR CCD camera work? lol the pixel elements are smaller than the wavelengths of the light they are interacting with! how does a solar cell work, optical wavelengths are far bigger than the size of the PN junction?

oh, another one: how do you produce optical frequencies? in LEDs the light emitting layer is thinner than the wavelength of of light! atoms emit light of wavelengths much larger than themselves all the time!

the details of how light interacts with matter at the microscale is dealt with in molecular spectroscopy for molecular systems and condensed matter physics for periodic systems. it is not easy.

16. Feb 18, 2013

### andrien

this fact is used to simplify calculation in quantum theory of radiation,in which a factor eikx is approximated by 1.this is called dipole approximation.

17. Feb 18, 2013

### DrDu

No, detector receivers (requiring no power source) using some magnetic coil antenna were quite popular.

Take also in mind that the first analytic treatment of absorption and emission was that of Heinrich Hertz who considered point dipoles.
The low efficiency in terms of radiated power is due to the fact that the radiated power depends on the absolute value of the dipole moment. The shorter the dipole, the higher the charges and currents have to be.
So the answer to your question is that the electric charges moved in electronic transitions in atoms and molecules are enormous as compared to that moved usual antennas. Try to calculate the transition dipole moment per mole of a substance and compare it to that of a metallic wire formed from 1 mole of the same substance of e.g. 1 m length and a voltage of 1V between its ends.

18. Feb 18, 2013

### Staff: Mentor

Or the long wavelength approximation!

Indeed, I think it is much more fruitful to think about the interaction in terms of frequency instead of wavelength. Consider that the atom is embedded in a uniform electric field which oscillates at frequencies that are commensurate to the typical timescales of electron motion (in a classical picture).

19. Feb 18, 2013

### NemoReally

Might it be helpful to consider that the conventional image of radio waves extends physically along the antenna, whereas the "waves" (which are better described as a bunch of phase-synchronized photons) are longitudinal? The "wavelength" isn't so much describing the physical length or extension of a photon but the distance (or time) in (our) space over which its E-M field "rotates".

What I found interesting was, taking a 10 MHz, 100 W tx, calculating how few photons pass through a half-wavelength antenna at 100 km in the duration of 1 cycle.

.............

I've used myself as an antenna on several occasions ... mostly for receive, but never at more than a few watts from the transmitter! Of course, I find I can shield myself from the CIA transmissions by using my tin-foil hat ... the Voices give good advice, occasionally.

20. Feb 18, 2013

### ZapperZ

Staff Emeritus
Actually, you didn't apply any logic. Your assertion that something smaller than the wavelength cannot interact with that wave is NOT based on logic. You cannot derive that from First Principles.

And considering that we have numerous experimental evidence to contradict that implies that your "logic" is faulty.

Note also that in metamaterials that have been shown to produce left-handed waves and cloaking, the structures that were built HAVE to be smaller than the wavelength of the incoming light. Now that light "sees" the structure as being "continuous" because the wavelength is longer than the size of the individual split rings and rods. However, the electrons in those structures certain DO interact with that light in such a way that it produced the necessary effects!

There is also something that you should try and learn here in this forum. When you think of something, you cannot solely rely on "logic", especially when it really isn't logical. This is science, or physics in particular. You cannot simply insulate yourself from experimental evidence and observation! So when you ask a question such as this, figure out FIRST if there are experimental evidence that contradict what you think you understand! Remember that just ONE valid experimental evidence is sufficient to falsify your "logic".

So when you think you have found that, based on some logic, light cannot interact with atoms, look for experimental evidence! Did Compton scattering never occur, for example?

Zz.