The definition of frequency is a measure of the number of occurrences of a repeating event per unit of time. It is also referred to as “temporal frequency.” The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency. Frequency is an important parameter in our understanding of waves and of wave-like phenomena, such as electromagnetism.

To calculate the frequency of an event, the number of occurrences of the event within a fixed time interval are counted, and then divided by the length of the time interval. In statistical work, it is more accurate to measure the time taken for a fixed number of occurrences, rather than the number of occurrences within a fixed time. This latter method presents—if N is the number of counted occurrences—a random error between zero and one count, so on average half a count, causing a biased underestimation of f by ½ f / (N + ½) in its expected value. In the first method, which is more accurate, frequency is still calculated by dividing the number of occurrences by the time interval. However, it is the number of occurrences that is fixed, not the time interval. An alternative method to calculate frequency is to measure the time between two consecutive occurrences of the event (the period T) and then compute the frequency f as the reciprocal of this time.

Stroboscope effect or frequency beats

In cases where the frequency is so high that counting is too difficult or impossible with the available means, another method is used, based on a source (such as a laser, a tuning fork, or a waveform generator) of a known reference frequency f0, that must be tunable or very close to the measured frequency f. Both the observed frequency and the reference frequency are simultaneously produced, and frequency beats are observed at a much lower frequency f, which can be measured by counting. This is sometimes referred to as a stroboscope effect.