Hi! I want to ask you something. Why on normal environments the [tex]\alpha > 0[/tex] (the coefficient of absorbtion) and in active environments where [itex]N_n > N_m[/itex], where [itex]N_n[/itex](the number of excited atoms) and [itex]N_m[/itex](the number of normal atoms) the coefficient of absorption is [tex]\alpha < 0[/tex]. I think in normal environments [tex]\alpha = 0[/tex]. !??