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, τ 21 is much longer than the lifetime of the radiationless 3 → 2 transition τ 32 (if τ 21 ≫ τ 32, 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 ν 21 can be obtained.