Frequency rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals (sound), radio waves, and light. For cyclical processes, such as rotation, oscillations, or waves, frequency is defined as a number of cycles per unit time. In physics and engineering disciplines, such as optics, acoustics, and radio, frequency is usually denoted by a Latin letter "f" or by the Greek letter "formula_1" or "ν" (nu) (see e.g. Planck's formula). The relation between the frequency and the period formula_2of a repeating event or oscillation is given by The SI derived unit of frequency is the hertz (Hz), named after the
Collision frequency Collision frequency Collision frequency is defined in chemical kinetics and collision theory, in the background of "theoretical kinetics". If a system contains "n" type-a molecules per unit volume and "n" type-b molecules in that same unit volume, then the number of a-b collisions per unit time in that volume will be proportional to the product "nn". or, in terms of molar volumes ("[x]=n /N" where N is the Avogadro constant) By kinetic theory it can be shown that: where The collision frequency ("Z") is usually expressed in units of "moles of collisions" per unit time per unit volume: or equivalently: