Frequency, f, is how many cycles of an oscillation occur per second and is measured in cycles per second or hertz (Hz). The period of a wave, T, is the amount of time it takes a wave to vibrate one full cycle. These two terms are inversely proportional to each other: f = 1/T and T = 1/f.

For example, if a wave takes 1 second to oscillate up and down, the period of the wave is 1 second. The frequency is the reciprocal of that, 1 cycle/sec, because only one cycle occurred in a second. If, however, a wave took half a second to oscillate up and down, the period of that wave would be 0.5 seconds, and the frequency would be the reciprocal, or 2 cycles per second. So, you see that a wave with a long period has a low frequency, while a wave with a short period has a high frequency.