Absorption

Photon with energy $E_{n_u}-E_n$ is absorbed by the atom in the state of n. At the same time the atom is excited from the state n to n$_u$. The number of this transition is proportional to the number of atoms in the state n, $N_n$, the number of photons passing and a cross-section of this absorption. This is written as

$$B_{n\rightarrow n_u} J_{\nu_{n\rightarrow n_u}} N_n,$$ (2.139)

where $J$ represents the average specific intensity of radiation with corresponding wavelength. This coefficient is called Einstein's $B$ coefficient for absorption.