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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:

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

, 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. For sound in air at 20 Celsius and atmospheric pressure, c = 343 m/s.

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

, where h = 6.626 x 10-34 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.