Planck constant (h)
The Planck constant (h) is a constant equal to 6.6256 × 10-34 joule second (6.6256 × 10-27 erg second). It scales the energy of electromagnetic radiation of frequency f so that the radiation appears only in quanta of energy nhf, where n is an integer. Planck's constant was first introduced in 1900, in Max Planck's theory of blackbody radiation, and subsequently appeared in Einstein's 1905 theory of photons. It also governs the accuracy with which different properties can be measured simultaneously (see uncertainty principle) and the wavelength of the wave associated with a particle (see quantum mechanics).