Since the atoms are also in equilibrium, the populations of the two levels are related by the Boltzmann factor : n 2 n 1 = g 2 g 1 e − − h ν ν / k B T where g 1 and g 2 are the multiplicities of the respective energy levels. Combining the above two equations with the requirement that they be valid at any temperature yields two relationships between the Einstein coefficients: A 21 B 21 = 8 π π h ν ν 3 c 3 B 21 B 12 = g 1 g 2 so that knowledge of one coefficient will yield the other two.