light-E.M.wave??

Discussion in 'Physics' started by pawankumar, Apr 9, 2011.

1. pawankumar Thread Starter Member

Oct 28, 2009
42
3
hi friends,
i ve a long time unanswered question and hope to get an answer here.

alternating current in a conductor produces electromagnetic waves whose frequency is the frequency of the alternating current in the conductor. What if we can produce alternating current with a frequency in the optical region, will the conductor glow by giving off light? what happens?has anyone done such research?

2. Wendy Moderator

Mar 24, 2008
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When you hit the frequencies involving light waves they are so fast that wires and electricity no longer work. Most light is made either with chemicals (think plasmas as in fire) or quantum effects (LEDs). The electricity provides power, but is no longer a direct part of the generating mechanism.

Microwaves, which are high frequency RF, are still a long ways from light frequencies. With the advent of nanotechnology this may not remain a fact, but for the moment my statements are safe enough.

3. magnet18 Senior Member

Dec 22, 2010
1,232
124
I had never thought of that..
do wires and electricity not work at those frequencies because the electrons physically couldn't move back and forth that quickly due to inductances and such?

4. Wendy Moderator

Mar 24, 2008
20,766
2,536
I think the closest answer is the inductance (and capacitance). There is also something with electricity and wires called the skin effect. Something I learned on this site is it starts in the audio frequencies, but gets more and more predominant with higher RF frequencies. All the AC signal rides literally on the skin of the conductor, so resistance effectively goes up too.

In short, by the time you get that high in the frequency chart electricity is effectively out of the equation. Maybe someone else can explain it clearer.

5. magnet18 Senior Member

Dec 22, 2010
1,232
124
Oh yea, I learned about the skin effect when I built an aluminum ring levitator (60Hz isn't a high enough frequency for it to matter much).

Thats why you use stranded wire for antennas.

I wonder how thin the wire would have to be... probably nano tech, like you said.

6. pawankumar Thread Starter Member

Oct 28, 2009
42
3
thank you friends..

7. mjhilger Member

Feb 28, 2011
119
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Although photons are described as electromagnetic, they are electrically neutral and have 0 magnetic fields. Photons (light) have definite frequency and are pure energy. They are created and absorbed as radiation (think heat like). When an electron has an abundance of energy and is in an outer orbit, where there is room on an inner orbit, as it decays back to the inner orbit a photon is given off, which is exactly equal to the energy between the two electron states.
So, if your wire were even theoretically capable of conducting electricity at those frequencies, in order for photons to be given off, the electrons would have to be in an excited state and continually moving between higher energy orbital state and lower orbital energy state and that would produce the light given off. If it were a perfect conductor, lossless in its transfer, even at that frequency, no light would be given off.
Energy of a photon is related to its wavelength by planks constant. And photons behave as dual concepts, waves and/or particles; as is expressed by the dual slit experiment. The energy being tied to the wavelength (color in the visible spectrum) is why blue LED's were hard to make. Finding the right semiconductor materials which had the correct energy levels between states to produce the blue wavelength was difficult.

8. magnet18 Senior Member

Dec 22, 2010
1,232
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Then are there sharp distinctions between where EM radiation can be created by current in a wire and where another form of creation is required?

9. mjhilger Member

Feb 28, 2011
119
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EM radiation is created by charged particles moving or changing magnetic fields. Photons are not charged nor are they magnetic. Different kind of mechanism responsible. But energy is transfered with EM radiation also. Atoms can be provided energy in many ways, heat, interaction by other atoms, excitation by other photons at the proper energy, etc. While we speak of EM, keep in mind that electricity and magnetic fields are different animals. They are related, Maxwell provides a context to connect the two, but (direct) detection and creation of the two require completely different techniques, think caps (electric field) vs. inductors (magnetic field) vs photoelectric effect(photons, but requires energy above particular level for that metal - called work function).

10. magnet18 Senior Member

Dec 22, 2010
1,232
124
So what is the maximum frequency possible to carry in a wire?

11. Wendy Moderator

Mar 24, 2008
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A photon is a single particle of an EM wave, they are aspects of the same thing. EM waves are how photons are handled as waves, and photons are how EM waves are handled with quantum physics. It is the duality of the photon. They are not separate and distinct.

Do a search on single photon experiments. A single photon exhibits the properties of a wave.

As nanophysics progresses there will be examples of wires handling those frequencies. One of many projects out there is the rectifier antenna, where nanoantennas at light wave frequencies receive light photons and rectify them to DC voltages. They are similar to solar cells, but use a different mechanism. Note the wires involved are below microscopic for this application. Their efficiencies are predicted to be high, but they tend to be tuned to one wavelength.

A quick search turned up this article (the concept has been around for a while)...

Flexible nanoantenna arrays capture abundant solar energy

I don't think there is a simple answer how high a frequency wire can go, as this is extremely technology dependent. Currently wire for light is fiber optic, and it is very analogous to wire. There is a transition area in high frequency microwaves where both conventional cable and wave guides are used. The cable is lossy, while the waveguides tend to be extremely efficient. Waveguides are hard to maintain though, as even a small defect (hole) will leak the microwaves like water.

Last edited: Apr 10, 2011
12. magnet18 Senior Member

Dec 22, 2010
1,232
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Could a superconductor handle higher frequencies?
Or are they still subject to the inductances and capacitances?

13. Wendy Moderator

Mar 24, 2008
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I don't know. Maybe. Even so there is going to be an upper limit.

How about superconducting nanoantennas? There are a lot of ways these technologies can go.

14. mjhilger Member

Feb 28, 2011
119
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In many microwave transmission and radar installations, the wire is one of the weakest links in the system, so rather than use wire to move the transmitter energy from the output stage to the antenna, a wave guide is utilized. So in that respect the upper limit of some of the wire used today is already at its upper limit.

15. mjhilger Member

Feb 28, 2011
119
16
A single photon exhibits properities as both a wave and a particle. This is the modern conundrum of quantum physics. The wave behavior follows the general quantum wave function, but is not manipulated in the same way as RF EM fields, as it cannot be steered via magnetic or electric fields. As in directional antennas for instance, a wave guide with TEM 00 (for a given frequency) steers the wave by forcing the walls to conduct and preventing waves other than 1/2 wave lengths from traveling, fiber cable only constrains the light to remain and has no outer electrical boundary, rather Snell's law dictates the reflective nature of the boundary of the glass cable and its outside media. This is because of the speed of travel (refractive index) of a given material.

16. Wendy Moderator

Mar 24, 2008
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I will respectively disagree, Electromagnetic Radiation and photons are the same thing. Read the definition from a source of your choice.

17. mjhilger Member

Feb 28, 2011
119
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Bill, the point I am trying to make is that the higher frequencies of light, while they follow the wave function, are effected differently by many of the mechanisms you indicated in metals. Photons hitting metal follows the work function and is absorbed, reflected, or transmitted. But those frequencies do not follow the same charistics as lower RF type waves as conduction in metals at that frequency moves electrons to higher orbits and is absorbed. The quantum mechanisms operate much differently because those frequencies interact with the material rather than see them as a hard surface, as is the case of the lower frequencies. If we take two EM fields at low frequencies and propogate them together, they will produce a combination of sum and differences and the two original. If we take two light sources and propogate them together, they will be seen as the mixture, but the wavelength of each photon is intact, they will not mix or interact with each other in the traditional sense, though you will see the wave mixing much as you see water particles mix to form waves on a lake. The wave front of the particles will display some of the characteristics, but the particles of photons (and their indivudual wavelengths) remain.

18. Wendy Moderator

Mar 24, 2008
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All of that is well and good, but photons are EM waves, and visa versa. I can quote several sources. Can you do the same?

Wikipedia Photon

Next search...
Next search...

These are the first three entries I pulled from the web using Wikipedia and Google. I did not pick and choose. Photons and electromagnetic radiation are one and the same. The only difference between RF, microwave, light, UV, and gamma is frequency. The fact each interacts differently with matter is fundamentally irrelevant, the particle or EM wave differs only in frequency. The interaction with matter only means you have to vary how you generate the photons according to the frequency required, which was the original questions asked.

This was my core subject in school way back when (Communications), then I hired on to Collins Radio. Some physics has changed around me, but not this (yet).

Last edited: Apr 10, 2011
19. mjhilger Member

Feb 28, 2011
119
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And classical Newtonian mechanics holds for large and small objects during a collision, but different things come into play within the quantum world. An electron colliding in a vacuum with another electron follows Newtonian mechanics, but it is a different story in a semiconductor within the lattice structure. The same holds true here. I am not trying to argue, but there are observable differences which experiments have proven over time and again. The dual slit experiment - photons behave as a wave if we don't look at which slit the photons pass through, but if we do look, they behave as particles. There are definite cases where the quantum world comes into play with EM waves and unlike the definite world of lower frequency waves, we can only calculate probabilities. My source is any quantum textbook.

20. Wendy Moderator

Mar 24, 2008
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Every one of those definitions were both current and included the quantum physics take. A third time I challenge you for a source. I've provided three. I've done my homework and cited sources.