Thanks. I guess I should have searched for “optical antennas.” :smack:
Well, now, this doesn’t sound right at all. All electromagnetic radiation is photons if you want to consider its particle nature, and it’s waves if you want to consider its wave nature. If you wobble a battery between your fingertips so that its poles, at different potential, are moving around, then you are emitting electromagnetic radiation in the form of photons. When charge circulates in an antenna to emit lower energy photons, electron energy transitions aren’t important. Photons are created by many different mechanisms!
All visible light is not created by electrons changing orbital levels. Line emissions generally are, but incandescence is blackbody radiation in a continuum, it’s not just so many lines run together.
The Sun is pretty much just hydrogen and helium, and they don’t have many emission lines - there aren’t many energy levels allowed - how does the Sun generate a pretty continuous incandescent spectrum from such a small set of lines?
Ah, er, um, an electromagnetic wave is composed of electric and magnetic waves at right angles to each other and to the direction of travel of the waves. Where does the magnetic field come from when you wobble a battery around? Wobbling a battery doesn’t create an EM field traveling away from the battery at the speed of light. It does result in an electric field at any given point that changes in accordance with Coulomb’s law.
You can regard a radio wave as being composed of a string of photons but they are of such low energy that that it isn’t convenient or useful to regard them as particles.
I’m sorry but black body radiation is explained by the elevation of electrons within the atoms of the material to a higher energy state and their subsequent falling back to a lower state releasing photons of various sizes.
A complete explanation of the sun’s radiation is beyond me. However, I will just say that the spectra of hydrogen an helium are not as simple a line structure as you seem to think.
If that were the case, then blackbody radiation would produce discrete lines as the energy levels in atoms are discrete. This is what we see in gas discharge lamps. Since light bulbs are not true black body’s, the spectrum is not entirely continuos and some electron recombination does occur, but it is not the main source of radiation.
From Wikipedia:
All electromagnetic radiation is caused by the acceleration of charge.
Long-wavelength light (radio or TV broadcast) can be generated easily by sending alternating current through building-to-person-scaled antennae. Perhaps you’ve been able to improve the reception on your TV by adjusting the antenna, only to see the signal degrade when you step away? In that case, your body was acting as an antenna for UHF or VHF waves, which have wavelengths of about 1 meter.
The microwaves in your kitchen have wavelengths of a few cm (that’s why you can get noticeable “hot spots” in microwaved food, where the waves add together). The magnetron in a microwave generates an oscillating field at a resonant frequency which is picked up and guided by a short antenna.
As pointed out above, and on MaxtheVool’s original question visible light from a “blackbody” source is a bit different: here, the acceleration of charge is not a resonant process but is distributed over many frequencies or wavelengths, like the light from the sun, which is mostly from the acceleration of charged particles due to collisions.
This kind of radiation is called bremsstrahlung (German for “braking”) and is a non-resonant process (i.e. it does not strongly favor one particular frequency or wavelength). Collisions between charged particles with a thermal energy distribution give rise to a continuum of frequencies. Napier’s example of the sun is a perfect illustration of this and ought to settle the matter of whether light has to come from atomic transitions, which are resonant processes: not only do hydrogen and helium have only a few distinct lines in the visible spectrum, but they are almost fully ionized in the solar corona, so atomic transitions cannot be responsible for the majority of the light emitted from the sun.
Incandescent filaments are also blackbody sources, with light coming mainly from non-resonant, collisional processes. The light from arc lamps and neon tubes, on the other hand, does come principally from particular atomic transitions, which is why those sources can have a dominant color, for example the characteristic orange hue of sodium lamps.
As has been pointed out, resonance at the high frequencies of visible light is difficult to create by some naiive implementation of alternating current. But if you think of the atoms themselves as antennae, then lasers are one answer to the original question. If you can pump many atoms into an excited state and then stimulate decay by sending another photon along at just the right frequency, you can double the intensity of the original photon. If you have a resonant cavity to select for that frequency, you can get very intense light at a single frequency without a filament.
Minor nit, and maybe it’s just my eyes, but Na lamps always looked yellowish to me. Neon, now THATS orange!
Solid material is not a gas and produces a continuos spectrum when the excited atoms relax from the high energy state.
Since the sun is icandescent acts close to a black body it would seen that thsun acts more like a solid body in this respect.
This statement is wrong.
To expand. Some recombination is occurring in an incandescnt bulb, that is the reason an incandescent bulb does not produce a pure continuum. Recombination does not produce a contimuum, it produces a line source. The primary source of radiation for an incandescent bulb is bremsstrahlung (illoe seems to know what he’s talking about.) radiation.
The Straight Dope on bremhstrahlung:
So this form of radiation has by far the greater part of its energy in the X-ray region and the electron must be given far greater energy than is available in a 120 volt incandescent lamp to penetrate past the surrounding cloud of other electrons and get near the nucleous and produce brehmstrahlung radiation.
On the other hand there is thermal radiation which is what takes place in an incandescent, or black, body.
Every source I can find attributes the light produced by incandescence to the relaxation of atoms back to a lower energy state after having been excited to a higher one by the input of heat. As is said in the quote above, solids and other dense matter act entirely differently that low pressure gasses because of the strong interaction between atoms and molecules in them.
Perhaps our disagreement here stems from our definition of orbitals, which are usually pictured as single-energy “steps” along an energy axis. Think of the Bohr atom. In this case, most transitions between orbitals are resonant and occur at very specific energies, and you’d never get a blackbody by just adding all the lines without scattering and transfer.
Solid state physics is different, as David Simmons points out: here, the atoms in a filament are so close to each other that their outer energy levels are destroyed and their valence electrons are free to roam about the ion lattice. Instead of energy levels, you have energy bands, which have a continuum of states between which transitions can occur.
But you still have to have collisions to enforce the distribution of electron energies within those bands in a way that gives you blackbody emission. (The microscopic origin of resistivity is the collision of electrons in the metal with ions in the lattice). And if you heat a filament to a high enough temperature, the band structure no longer really matters: yeah, you have transitions between states, but the states are in a continuum so they lose a big part of the “quantum” characteristics that most people think of when they talk about orbitals.
And a metal is like the plasma in the sun, insofar as it consists of positively charged ions surrounded by electrons which can move among them. The blackbody emission from both sources is still a non-resonant process dependent on collisions.
I think our disagreement has disappeared. I still don’t think that the radiation from an incandescent lamp is brehnstrahlung, but maybe that’s just a matter of terminology.
Solid state physics isn’t a strong point with me since it all happened, at least at the undergraduate and beginning graduate level, after I left school and you don’t need to know much about it to design equipment using solid state devices.
This little excusion has cleared up a little mystery that I have wondered about. I, too, wondered how you could get a continuous spectrum out of an incandescent material when electrons moved from one discrete level to another. However, with the idea of energy bands, voilà, the problem disappears.
So, I guess the answer to the OP’s question is, radio waves are the result of electrons being accelerated back and forth within a narrow location in the energy bands. Light is the result of a higher energy input moving the energy band to a higher energy state from which the atom, or molecure, relaxes back to the original level emitting light as it does so.
>Ah, er, um, an electromagnetic wave is composed of electric and magnetic waves at right angles to each other and to the direction of travel of the waves. Where does the magnetic field come from when you wobble a battery around? Wobbling a battery doesn’t create an EM field traveling away from the battery at the speed of light.
Just to clear this one up, a battery’s two ends are at different potentials - that is, they represent different amounts of charge. Not much, mind you, because most of their ability to do work is still chemical potential energy, but there is some nonuniform distrubution along, say, an AA cell. So, when you wobble it, there is charge travelling about, which creates a magnetic field. This has to propagate at c or some lower speed. Or said another way, when you wobble it, the charges at the ends are accelerating, which generates electromagnetic radiation.
Similaryly, playing badminton with an electrophorus emits electromagnetic radiation.
So does the spinning earth, because it has a magnetic dipole that doesn’t lie exactly along the rotation axis.