Visible Light and Other EM Radiation

We have machines which generate low frequency radio waves for broadcast, and machines which generate high frequency x-rays for medical purposes. How hard would it be to generate EM radiation in the visible spectrum. Wouldn’t it be cool to see green or orange emitted from an antenna?

Umm, Keeves?

They have such a thing. Thomas Edison invented an electric one. There were others based on wax and oil technologies prior to that.

We even have color specific ones. You’re probably looking at one now. My keyboard has three green LEDs, my monitor has a green one and a yellow one, my PC has several, including a red one.

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Well, you could just heat the antenna enough and it would glow red, but I don’t know if that’s what your getting at…

I’ve always wondered about something, which I’d like to append to the OP. Where is the cut-off point, if any, between antennas and cameras? I mean, we use antennas to pick up radio-frequency radiation, and cameras to pick up infra-red and visible radiation … is there some intermediate (far IR or microwave?) radiation that can be picked up by both antennae and cameras? Or picked up by neither?

The same would apply to the implicit question in the OP … if antennae can emit RF, and lamps can emit IR, is there some way antennae can emit visible (and IR), and/or some way lamps can emit RF?

And the same set of questions applies to high-frequency stuff. Antennae for UV? Cameras for gamma rays? Etc.

There are many common devices that emit EM radiation in the visible spectrum. They are called light bulbs. (Think of the filament as an antenna.)


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My understanding of light bulbs is that they simply get hot enough to glow and emit radiation in all sorts of random frequencies.

My understanding of a CRT monitor is that electrons are sent to the screen, and the pre-colored dots on the screen then light up.

I am looking for something like a radio broadcasting station, which can be adjusted to broadcast very specific frequencies, wavelengths, and amplitudes. Is that clearer? Thanks.

How about a laser then? Light comes out at exactly one frequency and wavelength.

Laser! Why didnt I think of that! Thanks!

Frequency is the inverse of wavelength. Visible light frequency can be specified by the color. A common light bulb can be considered a broadband low gain (visible) light source. A parobolic dish antenna with a narrow band source can be had by placing a color filter on a flashlight.
Of course, as stated above, a laser will provide a much narrower frequency spectrum. In addition, a laser has phase coherence (sp?) which is what gives it such a narrow beam width.

This is a fair question. The techniques for generating and using different frequencies do differ. You don’t generate opical frequencies with antennas.

When I was in grad school, ten years ago, they were just getting some millimeter wave equipment (waveguides and such) for experimentation. 60 GHz, IIRC. They’ve probably pushed the frequency they work with higher now.

I think there is still a gap between 100 GHz or so and IR (~10,000 GHz) that isn’t used yet, but this gap is closing, probably from both ends.

There are IR and optical IC lasers, used in fiber optic comunications. Last I knew, these (and LEDs also) had only recently (maybe 5 years ago) got to generate blue wavelengths. This gives more control than you’d have just heating a light bulb filament.

Xrays are generated differently yet again. I think there is another gap between visible and Xrays that doesn’t get used much.

Well, that’s kind of more than I know already… :eek:


It is too clear, and so it is hard to see.

One reason that you cannot make a wire antenna for light is very simple: Try looking down a piece of wire. Metals do not conduct light very well, so it would be difficult to get the electrons sloshing back and forth at the right frequency to generate sympathetic electro-magnetic waves in free space.

The other problem is that the length of the antenna is important. One popular antenna is 1/4 of a wavelength. The wavelength of light is so short that a 1/4 wave is shorter than a single atom of the antenna.

Hey KeithB, how do they do it with the really short-wave stuff, like X-rays? Can the antenna be several times (or several orders of magnitude) longer than the wavelength? Or do they use some completely different principle other than the antenna?

I confess complete ignorance on the beyond-UV stuff.

In a little-used corner of my brain I recall being taught that X-rays were generated by exciting atoms in the target element, stimulating X-ray emission. The term “K-capture” springs to mind – I think it refers to capturing electrons in the K-shell. The initial excitation is usually provided by an electron beam.

I don’t know of any specific “antenna” technology that is used. They just create the beam pointing in the right direction and block out the unnecessary parts.

: :pontificating::

BTW, the light generated by LEDs is created by laser action within the diodes. It is no longer coherent when emitted, however.

And while tunable lasers are available, most of them operate at a specific frequency governed by the properties of the elements involved. Even the tunable ones only operate over a very narrow frequency band. No laser exists that is tunable across the entire visible spectrum.

Finally, what is the difference, with regard to the OP, between generating monochromatic light with a laser and creating monochromatic light by filtering white light?

::/pontificating::

Yes,

X-rays are generated by exciting electrons and having them generate a photon. In fact, if you happen to have an electron microscope handy, one way to identify the substance you are looking at is to capture and measure the X-rays given off. Every element has a unique X-Ray signature.

You are correct, in effect every electron is a little antenna. But you will not be able to see “an antenna generating light,” which was the OP.

To answer the “appended” OP, somebody is working with waves from ELF, (10’s of Hertz) to gamma rays. The techniques for working with them vary smoothly as you go up in frequency, from lumped-component electronic circuits up to about 500 MHz -> distributed electronic components up to 20 to 30 GHz, to Quasi-Optical (A blending of distributed electronic and optical components) up to infrared to optical up to UV, to whatever they call the techniques they generate X-Rays with. (quantum components?).

You can use almost any technique at any frequency. I just read a paper where they were using several ton blocks of marble with holes drilled in them as acoustic lenses for audio frequencies.

In regards to X-ray production:

You are pretty much right – but there is a subtlety…

I don’t know of any device that produces X-rays by actually exciting an electron. The normal methods both involve a K-shell electron (the innermost shell is the K-shell).

K-capture: This is when a K-shell electron interacts with a proton in the nucleus creating a neutron. This produces a hole that really wants to be occupied by any electron it can get. Usually this is a valence electron – one of the outer electrons falls all the way down into the K-shell producing a fat photon that is usually in the X-ray region.

Same thing but not K-capture: An electron bean or something equally probing (it helps that it’s a negative beam) smacks a block of, say, iron. Occasionally, you’ll manage to knock out one of those K-shell electrons, thereby recreating the above situation.
Incidently, you can also knock out L-shell, etc., electrons, producing different energy photons.

Re the OP: The main hurdle has been stated already: an antenna’s size is of the same order as the wavelength radiation it produces ==> tiny antennae.

-P

To back up what KeithB said about the appended OP:

The trick to making a camera, as opposed to an antenna, is the lens. You can capture any kind of EM radiation you want, but you need a lens to focus it onto an array of sensors.

Oh, and I guess you also need a material thats opaque to the wavelengths you want, to shield the sensors so that they are only exposed in one direction.

(hmm, now that I think about it, maybe you can make a pinhole camera for any wavelength. So you really just need the opaque stuff. But pinhole cameras suck, so why bother)

Nobody ask what an “electron bean” is. :slight_smile:

(I hear they’re quite tasty.)

-P

OK, I won’t ask what an “electron beam” is.

Can you tell me about a cathode ray? 8^P

(For those who do not know, a cathode ray is an electron beam, which is way your TV picture tube, if improperly used, can generate X-Rays.)

Refried electron beans are particularly good with green salsa.

ZenBeam said:
When I was in grad school, ten years ago, they were just getting some millimeter wave equipment
(waveguides and such) for experimentation. 60 GHz, IIRC. They’ve probably pushed the frequency they work
with higher now.
According to Aviation Week, Jan. 10, TRW has developed a device that has operated at 69 GHz, the hightest speed ever recorded. It’s intended for electronic warfare use.