# Frequency

Glossary Definition for 16-19

## Description

Frequency describes the rate of repetition of a periodic event. Frequency is defined as the number of cycles (number of repeats) per unit time (the number does not have to be an integer).
Frequency is usually represented by the symbol f, or sometimes by the symbol ν (the Greek letter “nu”). For a periodic event with period *T*, the frequency *f* is related to the period by

*f* = 1*T*

## Discussion

Frequency and period are very closely related. In terms of measurement, it is usually easier to measure the period if the frequency is very low. The rotation of the Earth on its axis has a period of approximately 24 hours and we measure its period. But the vibration of a loudspeaker cone is of the order 10^{3} Hz and we measure the number of times it vibrates in a given time.

## SI unit

hertz, Hz

## Expressed in SI base units

s^{-1}

## Other commonly used unit(s)

none

## Mathematical expressions

*f*= 1*T*

where*T*is the period.- The frequency of a wave is related to its wavelength λ:

*f*=*v*λ

where v is the speed of the wave - For electromagnetic radiation with frequency
*f*, the energy*E*of a photon is given by

*E*=*h**f*

where*h*is the Plank constant.

## Related entries

- Period
- Wavelength

## In context

Electromagnetic radiation is often given a name according to its frequency and, sometimes, its application. Radio waves have frequencies between MHz and GHz. Visible light has frequencies between about 4.3 × 10^{14} Hz and 7.5 × 10^{14} Hz. Gamma rays have frequencies in excess of 1 × 10^{20} Hz.