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hboy007

Thu Nov 25 2010, 12:33PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
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Pinky's Brain wrote ...

Klugesmith wrote ...

Why would you call it a radio circuit instead of a lamp?

Because you modulate electron flows inside a circuit to create an EM field ... rather than bumping electrons (or photons) into molecules and using molecular oscillation phenomena to do it (which is what we usually do in lamps and lasers).

Well, at the scale and energies involved, we can no longer use the classical current flow model of free electrons flowing though a conductor. A small structure, correctly described by quantum mechanics, would have discrete states, electronic transport becomes a matter of symmetries and scattering processes, radiative dipole transitions become the quantum mechanical equivalent of dipole antennas but having to obey to certain selection rules, only certain transitions are significantly "allowed".

Steve Conner

Thu Nov 25 2010, 01:02PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
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Posts: 6706

Pinky's Brain wrote ...

Because you modulate electron flows inside a circuit to create an EM field ... rather than bumping electrons (or photons) into molecules and using molecular oscillation phenomena to do it (which is what we usually do in lamps and lasers).

It's the same thing! It's just that to make visible wavelengths of "radio", the "resonant circuit" has to be the size of an atom. And the only thing the size of an atom is an atom, by definition.

Pinky's Brain

Thu Nov 25 2010, 01:42PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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The structure could still be 100s of thousands atoms wide (assuming a metal lattice). It's small, but not that small.

Where does the energy for long wavelength photons come from (I always thought it simply came from a change in the kinetic energy of the conduction electrons as a group) and why couldn't the same mechanism create photons in the optical wavelength?

Steve Conner

Thu Nov 25 2010, 01:57PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I make it that a halfwave dipole for green light would be about 2,000 atomic diameters from end to end, not hundreds of thousands.

There are two kinds of radiation as far as I know, continuum and characteristic. Continuum just comes from random thermal motion of the electrons like in an incandescent lamp, and has the spectrum of black body radiation. Characteristic radiation comes from transitions between atomic energy levels, as in a laser, and comes at discrete frequencies characteristic of the particular kind of atom.

They don't correspond to long and short wavelengths: an incandescent lamp has shorter wavelengths in its output than a helium-neon laser, which in turn has a shorter wavelength than the continuum radiation from a lukewarm cup of coffee.

I don't really see any fundamental difference between a laser, a maser and a magnetron.

Pinky's Brain

Thu Nov 25 2010, 02:19PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Ya, sorry messed up ... still pretty macroscopic from a quantum point of view.

Proud Mary

Thu Nov 25 2010, 03:24PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
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Perhaps the boys have been trying to get at the idea of the Free Electron Laser.

Pinky's Brain

Thu Nov 25 2010, 03:54PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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AFAICS free electron lasers work essentially the same as antennas, except that the electrons aren't trapped in a conductor and moving on an additional axis.

Any way, it is possible to create optical antennas for reception. So if you can build the oscillator, which is a big if, transmission of light from a 250nm dipole antenna should be possible as well.

Adam Munich

Thu Nov 25 2010, 04:10PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
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Well who says the antenna needs to be a half wave dipole? It's always possible to use a physically larger full wave quad or maybe even larger multiples.

Also, why must the circuit be so small? Are you assuming each electromagnetic wave must make a full revolution?

Pinky's Brain

Sun Nov 28 2010, 01:27PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Grenadier wrote ...
Also, why must the circuit be so small? Are you assuming each electromagnetic wave must make a full revolution?

I was thinking that if the circuits was larger it wouldn't work because of radiative losses ... but since we are actually trying to radiate that's a bit silly.

PS. I'm just wondering ... is EM generation by antennas basically the same thing as stimulated emission synchrotron radiation?

Steve Conner

Sun Nov 28 2010, 03:16PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

All EM waves come from acceleration and deceleration of charged particles. That's the only way to make them. So I'd say yes.

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