Induced Emission

When atoms in the state n$_u$ are in the radiation field with the average intensity of $J_{n_u\rightarrow n}$, downward transition from n$_u$ to n is induced and atoms emit photons with energy $h\nu_{n_u\rightarrow n}$. This is called the induced emission or stimulated emission. This rate is expressed as

$$B_{n_u\rightarrow n}J_{n_u\rightarrow n}N_{n_u}.$$ (2.140)