# Frequency

From Citizendium, the Citizens' Compendium

**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.

The frequency ( * f* ) of vibrations is inversely proportional to wavelength (

*), which is a length measurement:*

**λ**- , where
is the speed of propagation of waves in the medium, or, for electromagnetic radiation,**c**= 299 792 458 m/s, the speed of light.**c**

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

- , where
= 6.626 x 10**h**^{-34}J•s (Planck's constant)