1.  = (4/3ħc) n em2  (Nm-Nn) (o-)
Square of the transition moment n em2
Frequency of the light 
Population difference (Nm- Nn)
Resonance factor - Dirac delta function (0) = 1

2.  = (4/3ħc) n em2  (Nm-Nn) (o-)
(Nm- Nn)

3. I = Ioe-γl
Beer’s Law

4. I = Ioe-γl

5. I = Ioe-γl  = (4/3ħc) n em2  (Nm-Nn) (o-) 1 2 3 4
Three Cases

6. I = Ioe-γl  = (4/3ħc) n em2  (Nm-Nn) (o-) 1 2 3 4
Three Cases
Nm > Nn

7. I = Ioe-γl  = (4/3ħc) n em2  (Nm-Nn) (o-) 1 2 3 4
Three Cases
Nm > Nn  +ve

8. I = Ioe-γl  = (4/3ħc) n em2  (Nm-Nn) (o-) 1 2 3 4
Three Cases
Nm > Nn  +ve I < Io Absorption

10. I = Ioe-γl  = (4/3ħc) n em2  (Nm-Nn) (o-) 1 2 3 4
Three Cases
Nm > Nn  +ve I < Io Absorption
Nm = Nn

11. I = Ioe-γl  = (4/3ħc) n em2  (Nm-Nn) (o-) 1 2 3 4
Three Cases
Nm > Nn  +ve I < Io Absorption
Nm = Nn  = 0

12. I = Ioe-γl  = (4/3ħc) n em2  (Nm-Nn) (o-) 1 2 3 4
Three Cases
Nm > Nn  +ve I < Io Absorption
Nm = Nn  = 0 I = Io Saturation

14. I = Ioe-γl  = (4/3ħc) n em2  (Nm-Nn) (o-) 1 2 3 4
Three Cases
Nm > Nn  +ve I < Io Absorption
Nm = Nn  = 0 I = Io Saturation
Nm < Nn

15. I = Ioe-γl  = (4/3ħc) n em2  (Nm-Nn) (o-) 1 2 3 4
Three Cases
Nm > Nn  +ve I < Io Absorption
Nm = Nn  = 0 I = Io Saturation
Nm < Nn  -ve

16. I = Ioe-γl  = (4/3ħc) n em2  (Nm-Nn) (o-) 1 2 3 4
Three Cases
Nm > Nn  +ve I < Io Absorption
Nm = Nn  = 0 I = Io Saturation
Nm < Nn  -ve I > Io Stimulated Emission

25. The maser may switch of after say 3 days so the dimension of the cloud can be estimated from the time for the light wave to pass through the cloud as about 1/100th of a ly and leave the molecules in their lower states. It will take several days to pump up to population inversion again and switch on again
Harry Kroto 2004