# Frequency

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Frequency is a property of an oscillation, vibration, or other regularly-repeating (cyclic) phenomenon. The frequency of such a phenomenon is the number of repetitions (cycles) in a unit of time. In the SI system of units, frequency is measured in Hertz (Hz), the number of repetitions in one second.

Frequency ( f ) is the reciprocal of the period ( T ), which is the time interval over which the phenomenon repeats:

${\displaystyle {f}={\frac {1}{T}}\,\!}$

For a wave propagating through space, frequency ( f ) is inversely proportional to wavelength ( λ ), which is a length measurement:

${\displaystyle \lambda ={\frac {c}{f}}\,\!}$, where c is the speed of propagation of the wave. For electromagnetic radiation in a vacuum, c = 299 792 458 m/s, the speed of light.

In electromagnetic radiation, the energy of the vibration is proportional to the frequency:

${\displaystyle E=hf\,\!}$ , where h = 6.626 x 10-34 J•s (Planck's constant)