Electromagnetic radiation includes radio waves, microwaves, infrared light, visible light, ultraviolet light, x-rays, and gamma rays. All of these types of radiation can be thought of as waves, like the ripples that spread out when you drop a pebble into a calm pool of water. An electromagnetic wave has a few important properties:

1. Speed: how fast is each ripple moving?
2. Frequency: if you point at the water with your finger, how many ripples pass by your finger every second?
3. Wavelength: the distance between two adjacent ripples.

The speed is easy. It turns out that all electromagnetic waves have the same speed, which scientists represent with the letter c. This speed, the speed of light, is equal to 670 million miles per hour.

The frequency can be any number. It is measured in Hertz, which means "One ripple per second." If two ripples pass by your finger every second, that's 2 Hertz. Most electromagnetic radiation has frequencies much larger than 1 Hertz. So we use larger units to measure the frequency:

 kHz kiloHertz, one thousand Hertz. (1,000) MHz MegaHertz, one million Hertz. (1,000,000) GHz GigaHertz, one billion Hertz. (1,000,000,000)

For numbers that are too large to express this way, we use "scientific notation", which is just a way of saying how many zeroes are after a number. For instance, 1 MegaHertz could be written 1E6 Hertz, meaning 1 with 6 zeroes after it. 2 GigaHertz is 2E9 Hertz. And 8,000,000,000,000 Hertz is 8E12 Hertz.

Here are some examples of frequencies of electromagnetic waves: *

 long-wave AM radio 200 kHz medium-wave AM radio 1 MHz short-wave AM radio 10 MHz FM Radio waves 100 MHz Microwaves in a microwave oven 2.4 GHz Infrared light 3E12 Hz Red light 4E14 Hz Green light 6E14 Hz Blue light 7E14 Hz Ultraviolet light 1E15 Hz X-rays 3E18 Hz Gamma rays 3E20 Hz

"Frequency" means the same thing for electromagnetic waves that it does for sound waves. In a sound wave, the frequency is the number of sound ripples that pass by in one second. For instance, the "A 440" note, which orchestras use to tune up, has a frequency of 440 Hz. The difference is that in an electromagnetic wave, the ripples are made of electric and magnetic fields, whereas in a sound wave, the ripples are made of moving air.

Both Astropulse and the original SETI@home use frequencies around 1,420 MHz, ranging from 1,417.5 MHz from 1422.5 MHz.

* In fact, each of these waves actually has a range of frequencies. For instance, FM radio waves actually range from 87.5 to 108.0 MHz.