In stimulated emission, what happens when an excited atom is struck by a photon?

• A. The atom emits a second photon identical to the first in frequency, phase, and direction of travel.
• B. The atom absorbs the photon preventing emission.
• C. The atom remains in the excited state without emission.
• D. The atom emits photons with random frequency and phase.

Answer: (A)

Stimulated emission happens when an excited atom encounters a photon whose energy matches the transition to a lower level. The incoming photon triggers the atom to drop to the lower state and emit a second photon that is a perfect replica of the stimulating one—same frequency, same phase, and traveling in the same direction. This coherence is what makes the light from lasers highly directional and phase-locked. If the atom simply absorbed the photon, it would move up to a higher level, which is not the process here. If the emission happened spontaneously without a trigger, the emitted photon would have random phase and direction. So the outcome is an emitted photon identical to the stimulating photon in frequency, phase, and direction.