The emission of photons by atoms, under the influence of the photons that comprise an electromagnetic field (light is essentially an electromagnetic field) applied to the atom, is a phenomenon called stimulated emission. Atoms also emit photons when an electromagnetic field is not applied, in a phenomenon called spontaneous emission.
Quantum electrodynamics shows that spontaneous emission takes place because there is always some electromagnetic field present in the vicinity of an atom, even when a field is not applied. Like any other system with discretely quantized energy, the electromagnetic field has a zero-point energy. Quantum electrodynamics shows that there will always be some electromagnetic field vibrations present, of whatever frequency is required to induce the charge oscillations that cause the atom to radiate ‘spontaneously’.
We can see that spontaneous and stimulated emission are qualitatively similar. In spontaneous emission, the electromagnetic field surrounding the atom is in its zero-point energy state. In stimulated emission an additional field is applied so that the electromagnetic field surrounding the atom is in a higher energy state. Then more intense field vibrations of the required frequency are present, and there is more chance that the atom will be stimulated to radiate.
From this argument, it is apparent that the transition rate for stimulated emission is proportional to the intensity of the applied electromagnetic field. For intense fields, it becomes very large and the atom radiates very efficiently. This has important practical consequences in the laser.