Recall from the descriptions of absorption and stimulated emission above that the rates of these two processes are proportional to the number of atoms in the ground and excited states, N 1 and N 2, respectively. If the ground state has a higher population than the excited state (N 1 > N 2), then the absorption process dominates, and there is a net attenuation of photons. If the populations of the two states are the same (N 1 = N 2), the rate of absorption of light exactly balances the rate of emission; the medium is then said to be optically transparent.