An electron in level 2 may decay by spontaneous emission to the ground state, releasing a photon of frequency "ν"12 (given by "E"2 – "E"1 = "hν"12), which is shown as the transition L, called the "laser transition" in the diagram. If the lifetime of this transition, τ21 is much longer than the lifetime of the radiationless 3 → 2 transition τ32 (if τ21 ≫ τ32, known as a "favourable lifetime ratio"), the population of the "E"3 will be essentially zero ("N"3 ≈ 0) and a population of excited state atoms will accumulate in level 2 ("N"2 > 0). If over half the "N" atoms can be accumulated in this state, this will exceed the population of the ground state "N"1. A population inversion ("N"2 > "N"1 ) has thus been achieved between level 1 and 2, and optical amplification at the frequency ν21 can be obtained.