v 11.13

$${\mathrm{d}I_f^\text{st}=hf \bigg( \frac{\mathrm{d}N_2}{\mathrm{d}t} \bigg)_\text{st} \mathrm{d}z=\frac{h f B_{21}}{c} N_2 I_f \mathrm{d}z}$$