Can antennas emit light? Light vs electricity - Page 1
Forum section: electronics / beginners

author: PsychoBoy
author_rank: Contributor
timestamp: May 15, 2019, 05:44:10 pm
content: Hi, so these questions are product of my silly thought experiments, sorry if can be just stupid, but I was wondering on generic light and electricity relationship and how it feels completely different. So I'd be glad if someone could help find answers to my questions:If antennas emit radio waves and they are electromagnetic waves, and light is also such a wave but the only thing that is different is wavelength then could I tweak my radio transmitter in such a way that antenna could emit visible light?Does electrons travelling through cable always emit electromagnetic wave in EM field? How about positive charged particles so called "holes", when they travel they also emit EM wave?If second question answer is yes then hitting end of the antenna is only to strenghten emitted wave? It's like ramping up speed of a car (electron) so when it hits a wall (antenna) it crushes wall (emits wave) strongly?If I think correctly then EM wave travelling in EM field from one side is generated by moving electrons but from second side has the opposite effect, when it encounters a cable with electrons it moves them and in this way it generated voltage wave back? It's feels like these effects are mirrored, electrons first emit EM waves in transmitter antenna and EM waves "move" electrons in receiver antenna so it "emits" wave back in the receiver cable.The "final boss" question is..if light is indeed EM wave and question four is correct (which is that EM wave is picked up by any cable as voltage wave) then doesn't light travelling in fiber optic cable also generate EM wave, I mean it IS EM wave, which can be picked by nearby cable? The question raised in my head from intuition that fiber optic cable doesn't interfere with electric signals and thus is best way for long data transmission, yet from my stupid thought experiments it feels like light in fiber optic cable is EM wave which can be picked by another normal electric cables and the opposite is also true, because let's say that electric supply cable to motor sits side by side with fiber optic cable, and huge current waves travelling to motor are generating EM waves which then CAN be picked up in fiber optic cable because LIGHT = EM wave..Yes I know I'm lacking some basic knowledge, although normal EE stuff seems so cut off from electromagnetism world it almost feels it isn't the same, so if someone could answer my questions and point me good book or source where I can understand electromagnetism then I'd appreciate it much.Cheers

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2019, 06:32:04 pm
content: Yes, light is electromagnetic radiation and so are radio waves. The movement of charged particles generates electromagnetic radiation.When a beam of light hits a piece of metal, it's true a tiny electric current, at the same frequency as the light is generated, but at these frequencies, it will be confined to the very surface of the conductor and won't travel far. In order for concepts such as capacitance and inductance to exist, the conductors much be much smaller than the wavelength of the radiation and red light has a wavelength of 660nm, so we're talking about under a tenth of that. Finally lots of metals are very lossy at optical frequencies and losses increase, with frequency. Above a certain frequency, a conductor will cease being a conductor. This is known as the plasma frequency.I don't have enough time to answer this in detail, at the moment. No doubt someone else will.

author: pwlps
author_rank: Frequent Contributor
timestamp: May 15, 2019, 07:06:37 pm
content: Your questions are not so stupid for someone without background in electromagnetism theory, in fact this kind of questions are asked sometimes by physics students during their freshmen EM course. But electromagnetism is very rich and full of pitfalls, it takes months and years to master them. If I had to answer fully your questions it would take me a whole book and I'm sure I would find some of the questions not simple to explain myself. So please don't criticise me if I just give a few short comments.1)QuoteYou would have to make your antenna very short, comparable to the wavelength. (Actually this is observed in surface-plasmon absorption effects observed in nanoparticles interacting with light).2)QuoteClassical electromagnetism only uses one parameter, the local charge density, which can be positive or negative. Any accelerated changein the local EM charge density/current will emit EM waves, therefore you can't make the difference between waves produced by electrons or holes (at very short distances there are quantum effects which can make the difference but you won't see it in any electronic setup).3)QuoteThe RF emission of the antenna is not due to "hitting the end" but just related to the time varying current in the antenna. At very high electron energies (short wavelengths) though, there is also a relativistic effect where EM radiation can be produced by "hitting the end" (https://en.wikipedia.org/wiki/Bremsstrahlung), this is the principle of X-ray production in vacuum tubes. (Yes, electromagnetism has deep connections with relativity, another potential difficulty).4)QuoteSomething like this, a mutual feedback between the charge distribution and the field is indeed contained in the set of coupled Maxwell equations: the charge density defines the field and the field determines the charge motion. But it takes a long time and many calculations to get a more intuitive picture of the equations5)QuoteThe light travelling in fiber optic cable also generate an EM field in the fiber, but it extends to extremely short distances from the interface (at optical frequencies the fiber glass behaves a bit like a conductor), much shorter than the fiber thickness.https://www.researchgate.net/post/How_to_mathematically_calculate_the_penetration_depth_of_skew_rays_in_a_optical_fiber

author: amyk
author_rank: Super Contributor
timestamp: May 17, 2019, 02:16:36 am
content: If you want to understand how EM radiation is used in radio and such, I recommend this very old but also very readable book:https://archive.org/details/principlesunderl00unit

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 17, 2019, 02:41:21 am
content: Indeed, so what's the difference?Another good question: you've seen pictures of YBCO superconductor, right? -- a black ceramic substance, which when cooled in liquid nitrogen or below, behaves really weirdly in the presence of a strong magnet (flux pinning, levitation, linear bearings..). Well, if it's a superconductor, and EM waves reflect off conductors, then, why doesn't it go perfectly shiny, or at least silvery instead of black, the instant it drops into LN2?The trick is: somewhere in the IR range, roughly speaking, the interactions stop being so much classical as quantum.This manifests in a number of ways, which ultimately arise from very deep physics. Obviously, quantum mechanics plays a role: this means EM waves stop acting so much as fields, instead as particles called photons, which are organizeden masseinto coherent waves. The wavelength becomes more like a particle size (but, it is of course more complicated than that; it's better to say, the waves are a probability density, a likelihood of detecting a photon at some time and space).The bulk properties of metals change noticeably. At low frequencies, you have the skin effect; at optical frequencies, you still have roughly the same behavior, but when you investigate closely, you find it's combined with weird phase shifts, very high indices of refraction (and also complex or negative values), and at still higher frequencies, entirely new, non-optical effects (e.g., photoemission of electrons).Indeed, metals are somewhat transparent. Hold a CD up to a light some time -- it looks dark blue, probably an interference effect (you'd expect it to look red, if it's due to skin effect). Light penetrates to a modest depth, 10s of nanometers. It is in this zone that visible light is reflected and absorbed by a metal!Dielectrics are usually surprisingly well behaved, give or take molecular effects (like IR absorption due to various kinds of atomic bonds and groups). A polyethylene cube has nearly flat behavior from DC to visible light. Water however does not.In fact, water exhibits a lot of quirky resonances and roll-offs and such, and practically by coincidence, has a transparent "hole" in what we call the visible spectrum. But, that should hardly be surprising; as any chemist will tell you, water isweird!There are some other practical considerations, too. Since we don't really have "metals" at optical frequencies (as mentioned, real metals act kinda weird), we can't make a classical antenna, and connect it to... some kind of oscillator, however it is you'd manage to make a classical oscillator at 500THz in the first place. We do, however, have molecules with conjugated or delocalized electrons -- fancy-speak for "conductive", at least in a local sense. These are characterized by alternating single and double carbon bonds, or bonds with other atoms that can share electrons. For example:https://en.wikipedia.org/wiki/Beta-Carotenehas 11 double bonds in a row, alternately. If we hand-wave enough, we can think of this as a resonant antenna, and as such, it absorbs some wavelengths characteristic of its length. If we shorten the chain, shorter wavelengths are absorbed, and vice versa. (But, at some point, harmonics will be absorbed too, as is the case for a classical dipole antenna as well.)And atoms and molecules can act as oscillators, given a source of energy (which is itself often another light source -- optically-pumped lasers, and phosphors, for example, but also chemical sources, like the blue glow of CH radicals in a flame). Active atoms can be coupled with active molecules, and the energy can be tuned or deferred or transformed in various ways.So, to answer the question more directly -- it's perhaps as important to ask "can antennas?", as "can antennas emit light?" That is, do antennas even exist, as such, at light frequencies? As you can see, such a seemingly daft question is actually quite an important one!TimP.S. To answer the superconductor riddle -- obviously, it simply doesn't work for light. But why? It turns out, the mechanism through which superconduction happens, has a very low energy level -- yes, it's a quantum mechanical effect, so it's a particles-acting-as-waves-and-vice-versa thing; and that means light of some frequency or higher, not onlycannotparticipate (i.e., be reflected by an induced supercurrent, as low frequencies can), but actually light isabsorbedand heats up -- disrupts -- the mechanism! In fact, thin film superconductors can be "switched" in this way (i.e., with a flash of light).

author: radiolistener
author_rank: Super Contributor
timestamp: May 17, 2019, 08:03:54 am
content: In order to emit light with antenna, you're need to feed antenna with 430 THz RF power.But it is almost impossible, because usual coax cable has too high loss on so high frequency.Just try to pass through light through coax cable, you will see it almost don't works due to high loss

author: radiolistener
author_rank: Super Contributor
timestamp: May 17, 2019, 09:04:41 am
content: Quote from: PsychoBoy on May 15, 2019, 05:44:10 pmNo, your transmitter is not designed to work on so high frequency, such as 430 THz. And you cannot tweak it, because 430 THz is out of specification for it's components.Quote from: PsychoBoy on May 15, 2019, 05:44:10 pmNo, not always. They emit EM waves only when accelerating or decelerating.When they move with fixed speed they don't emit EM wave.Electron and holes always have electric field, electrons has negative electric field and holes has positive electric field.When they stay at fixed point, their electric field also stay around them and it don't emit waves.When they moving at fixed speed, their electric field also moving with them and it don't emit waves.But when you're trying to accelerate or decelerate charged particle, it starts to interacts with their own electric field (because electromagnetic wave propagation speed is limited).This is why you're need to apply some energy in order to accelerate or decelerate charged particle.This known as electromagnetic mass. And this effect is a part of force which produce mechanic mass inertia.When you accelerate/decelerate charged particle, its electric field starts to move, and it leads to electric field density heterogeneity. This electric field density heterogeneity is EM wave, which flying away at light speed.It's the same when you trying to accelerate bobber in the water, you will see waves which flying around.The same thing happens with charged particle (bobber) and it's electric field (water).Quote from: PsychoBoy on May 15, 2019, 05:44:10 pmEM wave leads to move charged particle, the same as bobber (charged particle) moves under the action of waves on the surface of the water (electric field density waves). And vice versa.Quote from: PsychoBoy on May 15, 2019, 05:44:10 pmEM wave leads to move charged particles (free electrons in conductor) the same as wave on water surface leads to move bobber. Free electrons move is nothing else than electric current. So, EM wave leads to electric current in conductor.Quote from: PsychoBoy on May 15, 2019, 05:44:10 pmLight is EM wave. So, light cannot generate light, because it is already lightLight traveling in the optic cable is nothing else than wave of electric density which traveling inside optic cable. This wave don't fly away because optic cable walls reflect it back and keep it inside fiber cable in that way. The same thing happens in coaxial cable or waveguide.

author: soldar
author_rank: Super Contributor
timestamp: May 17, 2019, 12:00:29 pm
content: Where's that famous 1940s poster with the electromagnetic spectrum?

author: radiolistener
author_rank: Super Contributor
timestamp: May 17, 2019, 01:02:56 pm
content: This one?There are links to different resolution, up to 10000x6958 pixels:https://www.flickr.com/photos/llnl/9403051123/sizes/l/

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 17, 2019, 02:06:02 pm
content: the easiest is tip of antenna connected to + of light bulb, - to gnd, there goes your light emitting antenna.

author: radiolistener
author_rank: Super Contributor
timestamp: May 17, 2019, 02:24:05 pm
content: Quote from: Mechatrommer on May 17, 2019, 02:06:02 pmHere is another example of light-emitting antenna

author: golden_labels
author_rank: Super Contributor
timestamp: May 17, 2019, 03:40:02 pm
content: Quote from: radiolistener on May 17, 2019, 08:03:54 amIt’s just a matter of power. If it is high enough, it will pass and emit light in all directions. Along with some amounts of gases rich in carbon compounds.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 17, 2019, 04:50:46 pm
content: I seem to remember some research a while back that was using carbon nanotubes as optical frequency coaxial cables and antennas. I don't remember any details of what they were trying to demonstrate (probably: nanotubes!) or how successful they were. There is no reason it can't work, but it is really hard in practice due to all the reasons explained above.

author: windsmurf
author_rank: Frequent Contributor
timestamp: May 17, 2019, 05:18:18 pm
content: Quote from: Mechatrommer on May 17, 2019, 02:06:02 pmExactly what I was going to say... an incandescent light bulb IS a light emitting antenna. The filament does the conversion and transmission. So is an LED.