The fundamental constant of nature

(1)

arising in quantum mechanical problems. The quantity , called h-bar or sometimes the reduced Planck constant, is also commonly encountered. For example, the energy of a photon with frequency is given in terms of h by

(2)

Similarly, the Planck law for blackbody radiation is

(3)

where T is the temperature, c is the speed of light, and k is Boltzmann's constant. h also arises in the Schrödinger equation.

h-Bar, Photon