Thus a net power is released into the electric field equal to the photon energy hν times this net transition rate. In order for this to be a positive number, indicating net stimulated emission, there must be more atoms in the excited state than in the lower level: Δ Δ N > 0 . Otherwise there is net absorption and the power of the wave is reduced during passage through the medium. The special condition N 2 > N 1 is known as a population inversion, a rather unusual condition that must be effected in the gain medium of a laser.