Planck's constant (plängks)
Symbol h The constant of proportionality relating the energy of a photon to the frequency of that photon. Its value is approximately 6.626 × 10-34 joule-second.
A physical constant that is used extensively in quantum mechanics and fixes the scale of quantization of many phenomena, such as the relation between the energy of a photon (a quantum of light) and its wavelength. Its value is approximately 6.626 × 10-34 joule-seconds (equivalent to units of angular momentum). Planck's constant is fundamental to phenomena as the quantization of angular momentum and is used in Heisenberg's Uncertainty Principle. See also Dirac's constant, quantize.
A universal constant, first discovered by Max Planck, that states the mathematical relationship between the frequency of an electromagnetic wave and the energy in that wave. Planck's discovery unifies the seemingly contradictory observations that energy sometimes acts like a wave and at other times acts as if it is made up of particles.
Note: Knowing Planck's constant sets the scale of energy for events in which the atom and subatomic particles take part.