Discussion
A hypothetical material used was producing a laser beam was consisting of atoms having a three-level system with energies E1, E2 and E3 such that \(E_{1} \lt E_{2} \lt E_{3}\). The number of atoms in these energy levels were N1 in E1, N2 in E2 and N3 in E3, where \(N_{2} \gt N_{1} \gt N_{3}\). Laser emission will be due to transitions between the levels :
Answer: B
Laser emission is due to the \(E_{2} \to E_{1}\) transition.
Stimulated emission outweighs absorption on a transition only when the upper level is more populated than the lower one, a population inversion. Here \(N_{2} \gt N_{1}\), so the \(E_{2} \to E_{1}\) transition is inverted and can lase. This is the usual three-level scheme: atoms are pumped to \(E_{3}\), drop quickly to the metastable level \(E_{2}\), and lase to the ground level \(E_{1}\).
\(E_{3} \to E_{1}\) cannot lase because \(N_{3} \lt N_{1}\). \(E_{1} \to E_{3}\) is an upward transition, that is absorption (pumping), not emission, which rules out the options containing it.
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