The rate of absorption is thus proportional to the number of atoms in the lower state, N 1. The B coefficients can be calculated using dipole approximation and time dependent perturbation theory in quantum mechanics as: B a b = e 2 6 ϵ ϵ 0 ℏ ℏ 2 | ⟨ ⟨ a | r → → | b ⟩ ⟩ | 2 where B corresponds to energy distribution in terms of frequency ν. The B coefficient may vary based on choice of energy distribution function used, however, the product of energy distribution function and its respective B coefficient remains same.