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 organized
en masse into 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 is
weird!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-Carotene has 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!
Tim
P.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 only
cannot participate (i.e., be reflected by an induced supercurrent, as low frequencies can), but actually light is
absorbed and heats up -- disrupts -- the mechanism! In fact, thin film superconductors can be "switched" in this way (i.e., with a flash of light).
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