# Why is it that we never see visible spectrum waves being generated by oscillating currents in an Antenna?

Studying Maxwell's equations brings surprising clarity and odd questions to my mind. In this case, I was going through the Point form and Time Harmonic form of the four equations and a line in my textbook caught my eye. It goes like this "---time-varying fields or waves are usually due to accelerated charges or time-varying currents such as shown in fig(Figure shows a sine wave and a square wave). Any pulsating current will produce radiation (time-varying fields). It is worth noting that the square wave is a resultant of emitted radiation in digital logic boards. " Source: Principles of Electromagnetics by Matthew N.O. Sadiku (6th Edition)

In this context, I find it odd to notice and ask, why we have never seen oscillating charges produce em waves, like for instance an antenna. Granted, most antennas are designed to work at much lower frequency parts of the EM spectrum, but if for instance, we were to make an antenna, capable of generating signals, oscillating at frequencies corresponding to the visible spectrum, shall we be able to see the wave being generated?

And, if my question is stupid, I'd love to be told why... :)

• physics.stackexchange.com/questions/74892/… Apr 23 at 18:01
• We're trying: Optical antennas. Nano scale. Apr 23 at 18:25
• In order to have each wave reinforce the last, the antenna should be made to resonate at the frequency you're emitting. At optical frequencies this is the nanoscale Apr 23 at 19:04

why we have never seen oscillating charges produce em waves, like for instance an antenna.

You can experience this every day by looking at a mirror, which is just a flat polished conductive metallic surface. It is usually built on the back side of a sheet of glass for practical reasons: glass is transparent and it prevents the metallic surface from tarnishing due to oxidation.

The incoming EM wave wiggles the electrons in the conductor, which generates a counter-wave, which we call "reflection".

OK, that's not as exciting as a working 470 terahertz oscillator, but it still qualifies as a waveguide...

If the wavelength is longer than the thickness of a coat of paint, you can also replace the glass on your mirror with a coat of rust-proof paint, and it will still work.

EM waves as produced by antennas that are themselves visible (one centimeter to one kilometer long) are too low in frequency for your eyes to detect, which means you will never see them- as you point out.

The processes that create visible light photons occur on the size level of atoms and since your eyes do not have the spatial resolution to see an atom, you will never be able to "see" the emission of a photon by an atom.

Even if that were true, note that the process of photon emission will occur on a time scale of order ~1/(photon frequency) and you eyes can only resolve individual events occurring at a rate of ~1/30th of a second or slower. This means that even if you had eyes with enough spatial resolution to see individual atoms, they couldn't respond fast enough to capture the creation of a photon.

• Okay, so this seems to be a legitimate line of thought, which I'd like to explore with a bit more depth. So, tell me if I got it wrong, you're saying that the emission would take place, just that I won't be able to see it. Is that right? and, if that's what you mean, would Photodetectors, or similar receiving antennas, be able to detect those emissions? Apr 24 at 9:31
• All light signals you can see are emissions from atoms. In this sense you could describe a radiating atom as a little antenna, but I am not sure if this is what the OP was asking about. Apr 24 at 10:59
• Hardik, what I am saying is that your eye would of course detect the photon emissions but your eye could not observe in detail the processes occurring in the atom which created the photons in the first place. Apr 24 at 18:20
• Oh okay, got what you meant Niels. Thomas, I'm not really trying to relate the emission of photons through atomic processes, just through a technological pov. If an antenna can be designed to emit Microwaves or Radio waves, what would happen if we were to design an antenna, that could oscillate in terahertz(visible spectrum). Will it act as a luminous object at that point? would it emit visible light still? or do other physical phenomenons tend to step in and take over the process? Apr 25 at 9:28

You can do it with "plasmonic nanoantennas". I quote from this link

Out of all of the devices that emit visible light in a controlled way, plasmonic nanoantennas are the closest to traditional radio antennas. A plasmonic nanoatenna is a nanoscale, precisely shaped metal antenna that has plasma resonances excited in it (bunched-up electron oscillations). Since plasmonic nanoantennas rely on electrons that slosh back and forth between one point in space and another just like traditional radio antennas, thermal loss is still a major problem when they operate at visible light frequencies. For this reason, optical plasmonic nanoantennas are still laboratory oddities and are not practical sources of controlled visible light. Since lasers are becoming increasingly cheap, small, and reliable, there isn't really a motivation to develop plasmonic nanoantennas to emit information-carrying visible light. Furthermore, since electronic circuits can't run at optical frequencies, plasmonic nanoantennas can't be excited by hooking them up to an electronic circuit. They have to be excited by being hit with incident light. In this way, plasmonic nanoantennas aren't like traditional antennas at all. They are more like scattering objects.