What is stimulated emission?

• A. Excited stage by emitting two photons
• B. Emission of photons due to spontaneous decay
• C. Absorption of energy by an electron
• D. Propagation of light without energy transfer

Answer: (A)

Stimulated emission is when an incoming photon triggers an excited atom to emit a photon of the same energy, phase, and direction. The excited electron drops to a lower energy level, and a second photon is produced that matches the stimulating photon. This is why, after stimulation, two identical photons emerge in the same mode—coherent light is the hallmark, which lasers rely on.

Spontaneous emission, by contrast, occurs without any external trigger—the excited state decays on its own and emits a photon. Absorption is the opposite process, where energy is taken in to move to a higher level. Propagation without energy transfer isn’t a process that creates photons from excited states.