Coefficients used in the quantum theory of radiation, related to the probability of a transition occurring between the ground state and an excited state (or vice versa) in the processes of induced emission and spontaneous emission. For an atom exposed to electromagnetic radiation, the rate of absorption R_a is given by R_a = Bρ‎, where ρ‎ is the density of electromagnetic radiation and B is the Einstein B coefficient associated with absorption. The rate of induced emission is also given by Bρ‎, with the coefficient B of induced emission being equal to the coefficient of absorption. The rate of spontaneous emission is given by A, where A is the Einstein A coefficient of spontaneous emission. The A and B coefficients are related by A = 8π‎hν‎³B/c³, where h is the Planck constant, ν‎ is the frequency of electromagnetic radiation, and c is the speed of light. The coefficients were put forward by Albert Einstein in 1916–17 in his analysis of the quantum theory of radiation.