**Planck's constant**, denoted *h*, is a physical constant that is used to describe the sizes of quanta. It plays a central role in the theory of quantum mechanics, and is named after Max Planck, one of the founders of quantum theory. It has a value of approximately

A closely-related quantity is

**Dirac's constant**, after Paul Dirac.

Planck's constant is used to describe quantization, a phenomenon occurring in microscopic particles such as electrons and photons in which certain physical properties occur in fixed amounts rather than assuming a continuous range of possible values. For instance, the energy *E* carried by a beam of light with constant frequency *ν* can only take on the values

*ω=2πν*, which gives

*J*be the total angular momentum of a system with rotational invariance, and

*J*the angular momentum measured along any given direction. These quantities can only take on the values

_{z}

Planck's constant also occurs in statements of Heisenberg's uncertainty principle. The uncertainty in any position measurement, *Δx*, and the uncertainty in a momentum measurement along the same direction, *Δp*, obeys

## See also