hertz

The hertz (symbol Hz) is the SI unit of frequency. It measures the number of cycles or repeating events that occur per second. One hertz equals one cycle per second and applies to mechanical vibrations, electromagnetic waves, and other periodic phenomena.

It is named after the German physicist Heinrich Rudolf Hertz, who conducted foundational experiments on electromagnetic waves in the late 19th century. The hertz is a derived unit in the International System of Units (SI) and is used to express how often a given process repeats each second.

For angular frequency, the unit is radians per second (rad/s); these two are related by ω = 2π f. The frequency of a wave is related to its speed and wavelength by v = f λ in a given medium. Higher frequency typically means shorter wavelength.

Common prefixed forms include kilohertz (kHz, 10^3 Hz), megahertz (MHz, 10^6 Hz), gigahertz (GHz, 10^9 Hz), and terahertz (THz, 10^12 Hz). The audible range for humans is roughly 20 Hz to 20 kHz. Frequencies span applications from music and audio to radio, telecommunications, and optics, with visible light occupying hundreds of terahertz.