At the conclusion of Compton's 1923 paper, he reported results of experiments confirming the predictions of his scattering formula, thus supporting the assumption that photons carry momentum as well as quantized energy. At the start of his derivation, he had postulated an expression for the momentum of a photon from equating Einstein's already established mass-energy relationship of E = m c 2 to the quantized photon energies of h f , which Einstein had separately postulated. If m c 2 = h f , the equivalent photon mass must be h f / c 2 . The photon's momentum is then simply this effective mass times the photon's frame-invariant velocity c. For a photon, its momentum p = h f / c , and thus hf can be substituted for pc for all photon momentum terms which arise in course of the derivation below. The derivation which appears in Compton's paper is more terse, but follows the same logic in the same sequence as the following derivation.