# Generating Electromagnetic Waves

1. Dec 25, 2008

### nuclear420

When a current is accelerated through a wire, an electromagnetic wave is created. What determines the frequency of this wave? Is it possible (theoretically and experimentally) to achieve the frequency of light? (Possibly with a large voltage from multiple capacitors and a super-conducting material?)

2. Dec 25, 2008

### Staff: Mentor

The frequency of the radiated EM waveform is basically equivalent to the frequency content of the exciting current in the antenna. There are resonance and radiation resistance considerations, but basically what you put in (that's matched well to the antenna) is what you get out.

3. Dec 25, 2008

### nuclear420

What do you mean by frequency content? (An AC source or the material having a physical frequency limit?) Also, would it be possible to make visible light?

4. Dec 25, 2008

### nuclear420

I thought about the question some more and realized that if the current is high enough it will cause the free moving electrons to collide with the electrons in the stable atom and cause some of its electrons to be excited. Once these electrons de-excite, they can release visible light (or a frequency near it). (Example: light bulb)

As for free floating electrons producing light, the dielectric breakdown of a vacuum is a perfect example.

Now the only question unanswered is how the frequency of the EM wave is determined from the accelerated (or oscillating in the case of AC) current. Does the oscillation frequency of the current = the EM wave frequency? Is there a formula for it?

5. Dec 26, 2008

### ZapperZ

Staff Emeritus
Er..no. Your incandescent light bulb does not produce light that way. It is due to the heat generated by the vibrating ions in the lattice of the metal filament. This is why the light spectrum from such sources is continuous - try looking at it using a diffraction grating spectrometer.

This is not that big of a deal. As has been mentioned, the typical EM wave is generated by oscillating charges, NOT by collision with other things, unless you only want to generate bremsstrahlung radiation. In synchrotron facilities, the use a series of wigglers/undulators to cause the electron beam to oscillate and generate the various frequencies of light ranging from IR all the way to hard x-rays.

Zz.

6. Dec 26, 2008

### nuclear420

What about when radio waves are created by radio towers? You need specific frequencies for the specific radio channels. How would they ensure that their radio wave is created at only the frequency they need? How does it correlate with the oscillating current?

7. Dec 26, 2008

### Staff: Mentor

The size of the radio antenna is matched to (generally) multiples of 1/4 wavelength for the EM wave you want to launch. The radio station uses tuned oscillators at their broadcast frequncy (call it 98.5MHz as an example), and modulate that RF carrier with the audio or other information. The modulation frequency content is small compared to the frequency of the carrier wave (like you might modulate 20kHz audio onto the 98.5MHz carrier), so the frequency content of the power RF signal you are feeding to the broadcast antenna is still right around the frequency that the antenna is tuned for.

Here is a basic intro to broadcast antennas.