frequency

In physics, number of periodic oscillations, vibrations, or waves occurring per unit of time. The SI unit of frequency is the hertz (Hz), one hertz being equivalent to one cycle per second. Frequency is related to wavelength and velocity by the equation: f = v/λ where f is frequency, v is velocity, and λ is wavelength. Frequency is the reciprocal of the period T: f = 1/T

The electrical mains supply in the UK is AC (alternating current) with a frequency of 50 Hz. Each time the current changes direction, the voltage falls to zero before increasing again as the current moves in the opposite direction. At 50 Hz, the current will change direction 100 times a second.

At one end of the electromagnetic spectrum are long radio waves with a frequency below 10⁵ Hz and at the other extreme are gamma rays with a frequency beyond 10¹⁹ Hz.

Human beings can hear sounds from objects vibrating in the range 20-15,000 Hz. Ultrasonic frequencies well above 15,000 Hz can be detected by such mammals as bats. Infrasound (low-frequency sound) can be detected by some mammals and birds. Pigeons can detect sounds as low as 0.1 Hz; elephants communicate using sounds as low as 1 Hz. Frequency modulation (FM) is a method of transmitting radio signals in which the frequency of the carrier wave is changed and then decoded.