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• Harmonics

Any process that repeats on a regular basis has an associated frequency. The frequency is the number of repetitions, or cycles, that occur during a given time interval. The inverse of the frequency is called the period of the process.

Suppose you stand on a beach and watch the waves come in. You will notice that the waves arrive in a regular pattern, perhaps one every second. The frequency of that wave motion, then, is one wave per second. The period for the wave motion is the inverse of the frequency, or one second per wave.

All forms of wave motion have some frequency associated with them. That frequency is defined as the number of wave crests (or troughs) that pass a given point per second. Light waves, for example, have a frequency of about 4 × 10¹⁴ to 7 × 10¹⁴ cycles per second. By comparison, the frequency of X rays is about 10¹⁸...