The Planck constant (denoted h), also called Planck’s constant, is a physical constant reflecting the sizes of quanta in quantum mechanics. It is named after Max Planck, one of the founders of quantum theory. The Planck constant was first described as the proportionality constant between the energy (E) of a photon and the frequency of its associated electromagnetic wave (ν). This relation between the energy and frequency is called thePlanck relation or the Planck–Einstein equation:

E = hv 

Since the frequency ν, wavelength λ, and speed of light c are related by λν = c, the Planck relation can also be expressed as

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