Final answer:
An atom emits a photon during stimulated emission, often demonstrated when an excited electron returns to a lower energy state and releases a photon with a specific frequency.
Step-by-step explanation:
An atom is emitted during stimulated emission. In the context of an Atomic Frequency Comb and atomic transitions, this occurs when an atom that is already in an excited state absorbs a second photon and releases two photons as it returns to a lower energy state. Similarly, specific frequencies of photons are absorbed and emitted when an electron transitions to different energy levels within an atom. When a hydrogen atom absorbs a photon of light, an electron is excited to a higher energy level. The emission of light occurs when the electron returns to a lower energy state, and a photon is released in the process. The distinct lines in the hydrogen spectrum represent the unique frequency of light emitted as electrons transition between specific energy levels.