The Planck relation[1][2][3] (referred to as Planck's energy–frequency relation,[4] the Planck relation,[5] Planck equation,[6] and Planck formula,[7] though the latter might also refer to Planck's law[8][9]) is a fundamental equation in quantum mechanics which states that the energy of a photon, E, known as photon energy, is proportional to its frequency, ν: