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".