As I understand it, radio waves and light waves are basically the same things, on different parts of the spectrum.
Radio waves can be generated using radio broadcasters, which have no moving parts, seem like very simple, solid-state things, and don’t have “filaments” that burn out. Why can’t light be generated in the same fashion?
(Robert Heinlein posited this idea many years ago, and I’ve never understood why it’s implausible.)
It could if we could figure out how to make circuitry that can oscillate at the ~428 to ~750 terahertz needed to produce photons with the wavelength of visible light. Unfortunately, with current technology this is impossible.
LED’s are a form of solid state light. And fluorescent lights don’t have filaments, although neither works the same way as a radio transmitter in the way they generate electromagnetic radiation.
I suspect the amount of energy needed to create visible light by passing electricity through a conductor would be so large that it would not be all that different from a filament in an incandescent bulb.
Some of them do, but these filaments aren’t used to produce the light directly. Instead, they are used to heat up some mercury, turning it into vapor. The vapor is ionized and current passing through it produces UV light, which in turn excites phosphors lining the glass tube to produce various colors of visible light.
Great idea! I think I shall develop this into a product. I shall call it the “Light Emitting Diode”!
Seriously, similar devices, known as Gunn diodes are used in microwave transmitters. I’m unfamilier with current designs, but they were once used in every traffic radar.
Light is just too high frequency for the current state of semiconducter circuitry. For high power levels, microwaves are still generated using vacuum tube technology. The Magnitron in your microwave oven being perhaps the most ubiquitious example.
The reason that a light bulb produces light and the reason a radio antenna produces radio waves are fundamentally different. An electromagnetic wave is produced whenever charges are accelerated, and the frequency that the charges oscillate back & forth is the frequency of the wave. So to create a radio wave, we make a basically make a big loop of wire and cause current to flow in one direction and the other, at a very high frequency of oscillation.
A light bulb, meanwhile, works on a fundamentally different principle. As electricity flows through any wire, it meets a certain amount of resistance. The energy that’s lost to this resistance goes in to heat. If you pump enough electricity through the wire, it’ll get hot enough to start glowing; the filament of a light bulb glows for the same reason that a candle or a burning ember does, namely it’s extremely hot. Unfortunately, chemical reactions like oxidation take place more quickly at higher temperatures too; so we have to make sure that our filament is enclosed in some kind of inert gas so that it doesn’t react with the air in some way that makes it snap in two.
It would be theoretically possible, I suppose, to build a loop of wire and make the current in it flow back & forth at a sufficiently high frequency to produce light waves, rather than radio waves. Unfortunately, we really don’t know how to drive currents at such a high frequency (to get visible light you’d need the circuit to oscillate about 500 trillion times a second, compared to at most 100 million for radio waves.) So it’s just not possible with current technology, and seeing as there’s already a perfectly good way to produce light from electricity, there’s not much demand to develop such technology either.
There are nanocrystals which can emit light as well, which is getting pretty close. While still in the experimental stage, there are LEDs that implement nanocrystal layers, as well as some that are photoluminescent under certain conditions.
Radio transmitters having high-powered vacuum tubes have fillaments that do burn out.
LED’s don’t have filaments that burn out.
Although radio waves and light are the same thing, EM radiation, the radio waves are of exceedingly long wavelength as compared to visible light. The division point is fuzzy but short radio waves are in the millimeter range, say 1 millimeter or 1000 microns. The longest wavelength of visible light is about 0.76 microns. This means that a quantum of visible light is over 1300 times more energetic than a very energetic radio wave quantum.
This difference in energy means that light and radio waves are generated by different processes. Light is the result of exciting atoms to a high energy state and light is emitted when the atom “falls” back into a less energetic state. Radio waves result from the electric and magnetic fields generated when electrons are accelerated.
However, as has been pointed out, the generation of light doesn’t always involve filaments and the generation of radio waves sometimes does.
Maybe you left something out of Heinlein’s question?
>So to create a radio wave, we make a basically make a big loop of wire and cause current to flow in one direction and the other, at a very high frequency of oscillation.
>A light bulb, meanwhile, works on a fundamentally different principle. As electricity flows through any wire, it meets a certain amount of resistance. The energy that’s lost to this resistance goes in to heat.
But the heat is mechanical energy, vibration of the atoms in the filament, and vibration of the charge distributions. These bodies of charge going back and forth are what emit light. And that is actually fundamentally the same as in the radio antenna. They’re both charged bodies moving back and forth, just at different frequencies.
Well, not quite. The source of the light in an incandescent filament is not from the motion of the atoms. It is from the movement of the electrons within the atom from one energy level to a lower level. The energy lost in that change is energy level is radiated away as light, or infra red.
Yes it is still the movement of a charged body. However the electron that fell to a lower energy level might have been in that state for a long time before the change in energy. With radio, the radiation is immediate upon application of the alternating current.
If you can build the circuitry small enough- a few tens of atoms wide- and deal with all the ways that electron flow in such a small area differs from the flow in a macroscopic material, then it will oscillate at visible light frequencies. “Quantum dots” are one such approach.
>Well, not quite. The source of the light in an incandescent filament is not from the motion of the atoms. It is from the movement of the electrons within the atom from one energy level to a lower level. The energy lost in that change is energy level is radiated away as light, or infra red.
I don’t think this is so. Electrons falling through energy levels emit light in gas discharge lamps and LEDs, but thermally vibrating charge centers are what emit light and thermal radiation from hot glowing bodies. This is what makes infrared spectroscopy interesting - the harmonic oscillator behavior of a massive atom or group vibrating around its bond has a frequency characteristic of the mass and stiffness, and therefore of the bond. Isn’t this part of what Boltzmann got famous for?
It would have been nice had the author(Craig Freudenrich) mentioned that the idea of photons came from the work of Max Planck in his explanation of the spectrum of radiation from a black body.
In any case, visible light is a stream of photons.
How are photons produced?
And incandescence produces light by the same mechanism that a gas discharge tube does.
Infra red extends from just below the visible spectrum down to just above the microwave spectrum. At the shorter wavelength end the photons are produced by the same mechanism as for visible light. At some unspecified wavelength longer than that, the natural vibrational frequencies of atoms and molecules begin to generate radiation that begins to have the characteristics of radio waves. By the time the longest infra red wavelengths are reached the generation of the radiation is inseperable of the generation of radio waves.
The line between near infra red and light is arbitrary as is the line between far infra red and microwaves.
The line between IR and red isn’t arbitrary at all. It’s delineated by whether it’s visible to us humans or not. Now, I’ll grant you that the line is fuzzy, since not everyone has the same range of color vision, but it’s nevertheless well-defined within a fairly narrow range (I’ve seen anywhere from 700 nm to 780 nm, most commonly between 720 and 76 nm.) If you mean to say that assigning any one single value is arbitrary within that narrow range, I guess I can’t disagree with that
i suppose “arbitrary” was the wrong word. What I really meant was what you wrote here. There is a range of long wavelengths that various people can see so it’s not possible to point at a line in that range and say “this is the division point between visible light and infra red.”
Kind of related to this subject; I read something a while back about an experimental nano-fabricated antenna that was so small that it was tuned to ‘receive’ light in the same way a radio antenna receives radio waves - it was suggested that a surface coated with these devices (if that could be manufactured) would function as an exceptionally efficient solar power collector.