Planck constant
The Planck constant, or Planck's constant, denoted by h {\displaystyle h} , is a fundamental physical constant of foundational importance in quantum mechanics: a photon's energy is equal to its frequency multiplied by the Planck constant, and a particle's momentum is equal to the wavenumber of the associated matter wave (the reciprocal of its wavelength) multiplied by the Planck constant. The constant was postulated by Max Planck in 1900 as a proportionality constant needed to explain experimental black-body radiation.