To begin formalizing the correspondence between equation ( ) and the Planck-Einstein relation (i.e., 𝐸 * -𝐸 = 𝐸 em = ℎ𝜈 em ), consider the following postulate that is fundamental to this work. The postulate is that the energy of a single photon corresponds physically to the frictional energy accumulated during an orbital transition of an electron bound to an atomic nucleus. Therefore, the Reynolds number and friction factor are defined as