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If a radio could produce waves in the visible light spectrum, what would the result be?

This is a thought experiment that I've pondered for a few years now. I realize there are a few/many real-world constraints, but if we lifted these constraints for the sake of thought, what could we expect?

Personally, I don't see why we wouldn't observe visible light emitting from the antenna, disregarding any light from Blackbody Radiation.

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I am not sure I understand the question. If a radio emits visible light, then why wouldn't you see it? Careful with all these questions starting with "If something was doing something impossible", you often obtain impossible results. –  fffred Aug 22 '13 at 6:55
    
First, I'll apologize for the double negative. What I was trying to say was I would expect to observe visible light emitting from the antenna. Secondly, if I thought this was possible, I would have left out the words "thought experiment." I realize this question is 100% hypothetical, and apologize if hypothetical discussions are discouraged in these forums. –  Austin A Aug 23 '13 at 2:55
    
I have wondered that as well. I suspect if we could do this without any constraints getting in the way, then we would have an excellent source of any color you can create a frequency for. Emitting that color in a spherical wave front. –  M0J0 Feb 5 at 11:25

5 Answers 5

up vote 9 down vote accepted

You might want to have a look at Does light induce an electric current in a conductor?. It's probably impossible for a radio aerial to emit visible light as the frequency of light is around the plasma frequency of the metal that the aerial is made of.

We're not really supposed to address hypothetical questions, but if you could find some material with a high enough plasma frequency (remember this applies to everything in the circuit) then the emitted light would just be light. You'd see the aerial glowing.

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what is hypothetical here? OP is quite good question. –  Asphir Dom Aug 29 '13 at 19:58
    
@AsphirDom: it's difficult to see how you'd construct an electrical circuit carrying a signal with a frequency equal to that of light. Conventional conductors wouldn't do because their plasma frequency is too low. –  John Rennie Aug 30 '13 at 6:00
    
Great answer! I'm new to posting to the forums so sorry for the type of question. I just wanted to provoke thought, that's all! –  Austin A Aug 31 '13 at 2:33

This isn't hypothetical. There is nothing that a radio does that can't be done in other parts of the spectrum. Many FM/AM radios operate in the optical range too. Your TV remote control uses IR. Lasers are used for high bandwidth point to point communications. And don't forget fiber optics, these are all radios that just use optics for the communication path.

Using visible light is possible too, but there is a lot of noise in that band, so it isn't practical unless it is confined to a fiber or a clear channel path.

If you could "see" a FM radio's modulated output in the visible spectrum it would just be a blur of color (appearing mostly white if centered on the visible spectrum) because it modulates colors faster than you could accurately perceive them with your eyes. AM would just look sort of dim like an LED blinking really really fast. See this fiber optic video.

If you want to make your cell phone transmit in the visual range, all you have to do is detect the data (in a receiver) and upconvert the data by AM modulating a LED diode (off and on). If you want FM like modulation, then you have to use something that is easier to shift the frequency of like a fiber optic transmitter.

You can't easily get a piece of metal to emit light like the type of radio you are thinking of in your question, however you can get other structures to easily emit light and it is done all the time.

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Thanks for not denouncing my question right off the bat! I like how you brought up noise though. Didn't think about that when I initially conceived this question. –  Austin A Aug 31 '13 at 2:32

the power applied by transmitter makes the electrons in the antenna material accelerate or decelerates with the applied frequency . Electric charges radiates e.m radiation when they are accelerated

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I like thought experiments ! No need to replace those expensive melted bits !! And that is relevant to this experiment. We will wind up the RF output of the transmitter until the aerial itself is glowing a nice cherry red . Still intact (just !) as a functional aerial but emitting light (you can see its red). So assuming we have still got a good SWR and all the aerial current is still true RF the question is are we seeing a pulsating red colour (at the RF frequency ) ? I am just going into the living room to conduct a thought experiment on my vintage 1 bar electric fire by winding the frequency of the mains supply down to 0.5Hz. Results will follow later (I think ).

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I suspect Austin was just wondering what it would look like if we could see radio waves. The best analogy I can think of to represent that is to put an arc light from a carbon arc search light without the shroud and reflector on the top of the antenna pole and apply the same wattage. The main difference is that the electromagnetic field of the radio wave propagation would depend on the antenna design, where the arc would be pretty much spherical.

Austin, was this what you wanted to know?

By the way, that much power from a non-focused arc would cause serious safety issues and could bring a lot of eye-damage law suits.

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protected by Qmechanic Feb 5 at 16:21

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