Light from Distinct Electric and Magnetic Fields?

Islam Hassan

If we set up an experiment in a chamber with distinct perpendicular oscillating i) electric and ii) magnetic fields, cycling at a frequency representative of visible light, will the chamber suddenly light up? If not, why?


IH

mfb

The creation of those fields is the emission of light, so... yes.

Islam Hassan

Wow! Has this been done experimentally or is it too difficult to tweak/fine tune the two fields?

IH

mfb

Lasers do something similar. If you want to use electronics to generate those fields, you are (currently) limited to lower frequencies like Masers or RF cavities.

Khashishi

You don't need to supply both the electric and magnetic fields. If you create an oscillating electric field, the oscillating magnetic field will form automatically. That's what an antenna is.

sophiecentaur

Lasers etc. are still quantum devices, though and the emission is from atoms - hard to know what they are doing when radiating, in terms of the fields.

There are all sorts of things that you can do with fields at (the much lower) Radio Frequencies by manipulating the actual fields involved rather than taking the photon approach.
It is possible to take a coil and two flat plates or a short dipole (both structures significantly smaller than the wavelength in question) and feed them with the same signal in appropriate phases. They will produce oscillating (predominantly) magnetic and electric fields, respectively in the near vicinity. However, each one, taken on its own, will also produce the 'other' field and will radiate in its own right, when you look in the 'far field'. But by using both together, you can produce the equivalent of the far field wave closer to the structure and you will actually be able to radiate more power than each element taken separately if they are both small compared with a wavelength.

Islam Hassan

What frequencies does that cover? If you had the electronics that could feed light frequencies to some suitable form of antenna/radiating device, could it have any practical applications? Would it generate light much more efficiently than light bulbs (no heat)?


IH

sophiecentaur

Why should it? An LED does pretty well and it can only be a matter of time before all lighting is with Quantum devices but atoms 'do their own thing' and don't produce separately controllable E and H fields. The only time you can play with individual fields is when the photon energy is so low that you can tinker with the impedances of the radiating systems. The microscopic structure of atoms and crystals is far harder to manipulate than just winding coils or altering the length of a wire.
You seem to be leaping from "is it remotely possible?" to "would it be more efficient?". If the answer to the first is doubtful / impossible then how could it be more efficient?
Beware falling in love with wacky ideas. They can easily lead up blind alleys.

MarkoniF

What is the limit when trying to produce visible light with electronics? Or in other words, what does it take to make radio antenna glow with visible light?

sophiecentaur

Quantum mechanics, basically. Your radio antenna would need to be around the wavelength of light, it would need to conduct charge and that charge would need to be moved by the presence of an alternating emf at 10^12Hz. It would be non-trivial to make an 'inductor' of that size in a material that would behave like a metal conductor. Unfortunately, the photon energy of visible light is a lot higher than for regular radio frequencies (millions of times more than even for microwaves). Let's face it, the best source of optical frequencies is electrons in various energy levels with respect to positively charged nuclei and I think you'd be basically building one of them!

MarkoniF

I'm not sure why, but that doesn't sound impossible at all. How close we are, what's the best we can do, these days?

sophiecentaur

Umm. I love your optimism but I think it's more basic than that. But how much 'like a radio transmitter' would this device need to be in order to satisfy your requirement? It's pretty easy to get a piece of metal to 'glow' but I suspect that wouldn't be good enough.

When you try to generate alternating currents, at frequencies that are currently attainable, how you do it will depend upon the particular frequency involved. For instance, no one wouldn't make an LF radio transmitter using a Klystron Amplifier. So it need be no surprise that entirely different technology would be used to produce light compared with what's used for RF. Masers and Lasers both work in a similar way but they are essentially Quantum Devices so they aren't 'radio circuits' and the radiating elements are more 'optical' than dipole based.
I think that there is an essential difference / 'gear shift' when you move from classical to quantum devices with a bit of overlap in the mm microwave region and I can't see why that would change. It isn't just a matter of scale. But I guess there would always be the possibility that someone would be wanting to push the boundary by an octave - just because it's there.
Whilst you're about it, why not suggest a gamma ray oscillator circuit -haha.

mfb

The inverse direction is known as nantenna (and somewhere between hypothetical and possible) - if those can be constructed, the reverse direction might work as well.

Islam Hassan

lol, no-one is falling in love with anything here...the simple idea that one can theoretically generate light from pure electric and/or magnetic fields is fascinating, which doesn't mean that it's practical. My question was couched in the conditional and it's not like I'm planning to spend my life's savings on trying to make the idea practical. Physics is fascinating in its own right and doesn't need a passionate devotion to wackiness to enhance its enjoyment...


IH

Islam Hassan

Thanx for the reference, very interesting concept.


IH

jtbell

Correct. I'd state this more strongly: you cannot create oscillating electric and magnetic fields completely independently, and then "combine" them. When you create an oscillating electric field, you must get an oscillating magnetic field along with it, according to Maxwell's equations. Similarly, if you create an oscillating magnetic field, you must get an oscillating electric field along with it.

sophiecentaur

I think this thread is really concerned with stretching the Classical regime over into what has, up till now, been the Quantum regime. I guess the boundary is fairly fuzzy and could stretch along with our inventiveness.

Not sure that 'the other way round' is feasible. As a 'receiver', the nantenna is a very clever passive device which they tell us can be more efficient than a PV cell (there must still be a small 'diode drop' or equivalent somewhere, to cause the rectifying action, though it will be much less than a conventional diode). (And the frequency doesn't reach the optical bands yet) As a simple rectifier, it is strictly a 'downhill' device - producing DC from AC. As a 'classical' device, it's definitely a step further than just using the thermal effect of received radiation as in a thermopile or radiometer.

Producing an alternating PD, in the classical sense (which is what you need for any transmitter) is a harder task and needs an amplifying component to switch or modulate a direct current. An atom, raised above its ground state by a DC power source with a beam of electrons in a discharge tube, does just what you want - one photon at a time. Electrons in the junction of an LED do the same. But this is a Quantum effect and not within your rules.

I was wondering about LED efficiency and I just found this link about LEDs with 'over unity' efficiency - in terms of electrical power in and light power out. They take heat from the surroundings to make up the balance. Now that's the sort of efficiency that you can really only expect to get from a Quantum Device.

BTW, would synchrotron radiation be a suitable candidate for your idea? You can take a beam of electrons and make them follow a tight enough curve. That will produce EM of any frequency you want.