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Q & A: Extreme parts of the electromagnetic spectrum

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Most recent answer: 10/22/2007
Q:
Is there a frequency of light wave lower or higher than gamma rays / radio waves, and how would you detect it?
- Rob (age 15)
Meols Cop High School, UK
A:
Well, this gets more into the definitions of words, as you can call a very very low frequency wave a "radio" wave if you want to. Waves with a frequency of just a few Hertz are called "ULF" waves -- Ultra-Low-Frequency waves, and are used to communicate with submarines, so they perhaps can be classified as radio waves. The electrical power in the U.S. gets distributed at 60 Hz (50 Hz in Europe), and so the power lines emit electromagnetic waves at 60 (50) Hz. Radio cannot use that frequency for two reasons -- there's too much interference from power distribution, and the kind of stuff you want to broadcast uses a wider spectrum than the carrier will allow. Music has frequencies from about 100 Hz to maybe 5000 Hz and putting that on a 60 Hz carrier won't do the trick. You can do Morse Code or something like it with the ULF waves of course. You can detect these very low-frequency waves in the same way as radio waves -- you just need a very big antenna.

The same goes for the high end of the spectrum. I'm a high-energy physicist, and we routinely detect single photons with have energies of 50 GeV or so. 50 GeV is the amount of energy an electron would gain if it went across 50 billion 1V batteries in series. We call these gamma rays, and do not have another name for them as the energy gets ever higher. At these energies, the best way to detect them is with what we call a "calorimeter". You make them typically out of stacks of lead plates with scintillator material sandwiched in between. An incoming photon with this energy has so much energy that when it hits a lead atom, it is able to "pair-produce" an electron and an antielectron, both with high energies. Then the electron and antielectron fly past more lead nuclei, decelerate and radiate more high-energy photons, which can produce more pairs of electrons and antielectrons. At high energies, thousands of such pairs can be made in a "shower". Some plastics will emit flashes of light when high-energy particles travel through them. Then phototubes are used to detect the flashes of light, making electrical pulses which can be amplified and recorded on computers.

Tom

(published on 10/22/2007)

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