# Frequency

**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 (

*), which is the time interval over which the phenomenon repeats:*

**T**For a wave propagating through space, frequency ( * f* ) is inversely proportional to wavelength (

*), which is a length measurement:*

**λ**- , where
is the speed of propagation of the wave.**c**

For electromagnetic radiation in a vacuum, * c* = 299 792 458 m/s, the speed of light. For sound in air at 20 Celsius and atmospheric pressure,

*= 343 m/s.*

**c**In electromagnetic radiation, the energy ( * E* ) of a single photon is proportional to the frequency:

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

For waves encountered in everyday life, typical frequencies are 20 to 20,000 Hz for audible sound, 1 MHz for AM radio waves, 100 MHz for FM radio waves, and 430 to 750 THz for visible light.

Non-wave phenomena can also be periodic and have an associated frequency. Household electricity is typically either 50 or 60 Hz, depending on location. A car engine running at a modest 1000 rpm has a 17 Hz frequency. Earth's once-per-day rotation corresponds to about 1 milliHz, and its once-per-year revolution about the Sun corresponds to about 3 nanoHz.