# Can I use an antenna as a light source?

Can I use a normal metal antenna to emit visible light?

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Although I wouldn't say this is necessarily a bad question, it's not an ideal example of the kind of thing I'd like to see on the site and I'm very surprised that it got so many upvotes. –  David Z Feb 13 '11 at 6:26
antennae dimension needs to be of the order specifically $\lambda /4$ of the wavelength to be emitted. For visible light it is ~ nm. SO one need nm scale antenna for it to happen theoretically... –  Vineet Menon Apr 9 '12 at 9:05
Maybe its also possible to use a Harmonic Antenna? An antenna which has the length in an integer multiple of the wavelength? –  user9754 Jun 9 '12 at 12:00
to nanometer accuracy? maybe a single crystal. –  anna v Jun 9 '12 at 15:44

with your question you tackle one of the most rapidly evolving fields at the moment: Nanophotonics. Let me explain in the following why this is indeed the case.

## Dipole antennas

It was Heinrich Hertz, student of Kirchhoff and Helmholtz who tried to verify Maxwells equations by generating electromagnetic radiation building a dipole antenna out of a somehow transformed RLC-circuit:

This is the physical ground on which we have to investigate your question to get an idea if optical antennas are possible and if you could use some of your ones to create red light. So let's look at the characteristic value of such an antenna: its length.

## Length Scales

Roughly speaking, to be able to generate radiation with the mentioned form of antennas, one needs to impose a standing wave on the antenna. Assuming that the reflection at the ends of the antenna is somehow perfect, we find that an antenna will have a length of approximately $$L \approx \frac\lambda2\,.$$

If you look at the electromagnetic spectrum,

red light has a wavelength of about 600 to 700nm, your antenna will have $$L\approx 300nm$$ which is incredibly small. This is the reason, antennas in optical frequencies are subject to current research, see e.g. Resonant Optical Antennas by Mühlschlegel et al.

So, from this point of view it seems very unlikely that a Layman will be able to produce an antenna that can emit light at optical frequencies.

Sincerely

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+1 for the images :) –  Marek Feb 12 '11 at 10:12
In addition to my post I explained how nanoantennas can be used to access and control quantum systems, in case anyone wants to read further :) –  Robert Filter Jan 7 at 10:08

Yes, if you heat it :) But assuming you want to use it in the usual way the answer is no.

The color of light depends on its wavelength (assuming it's monochromatic; otherwise it will be some mixture of its components). Normal antennas have length scale on the order of centimeters to meters which is somewhat longer than microwave radiation which is still very much outside the visible spectrum.

To get visible light, you need wavelengths on the order of 100 nm (and specifically for red light around 600 nm). Where do you get such short lengths? Well, light carries energy inversely proportional to its wavelength. It turns out that these energies corresponding to visible light are often present in the absorption/emission energies of usual atoms (which is ultimately why some objects look red). Of course, this argument should in fact run backwards: the fact that our eye responds to visible light is because that's the kind of radiation to be found around us. But I digress.

And as alluded to in the beginning, you can also get red light by heating stuff. This is because at certain temperature the thermal energy will be just right to produce (mostly) red light.

In any case, to produce red light (in particular coherent, as in lasers) you need to use the above observation about absorption and emission and base your device on semiconductors, crystals, etc.

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Ok, Marek, you have been two minutes faster but I have two more images ;), +1 for the heating. Greets :) –  Robert Filter Feb 12 '11 at 10:05
To digress/nitpick more: Partially heated objects looks red because most of the radiation is infrared which we can't see. –  Macke Feb 13 '11 at 10:06
@Marcus: good point. –  Marek Feb 13 '11 at 13:25
+1 for the red heat :=) –  Georg Dec 5 '11 at 19:47