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Intensity of spectral lines, , * Transition probability: likelihood of a system in, , one state changing to another state — decide, whether transition forbidden or allowed —, selection rule, , Population: number of atoms or molecules, initially in a state from which transition occurs —, greater population, large intensity —, , N2/N1 = e (E2-E1)/kT = @-AE/kT, , Concentration/path length: amount of material, present giving rise to the spectrum- more path, length, more energy absorbed