In stimulated emission, which property do the emitted and stimulating photons share?

• A. Photons with different wavelengths
• B. Photons with random phase
• C. Photons absorbed
• D. Photons identical in frequency, phase, and direction

Answer: (D)

Stimulated emission happens when an incoming photon of the right energy interacts with an excited atom and causes it to emit a second photon into the same electromagnetic mode. Because the emission is driven by that existing wave, the new photon ends up with the same frequency (energy), the same phase, and the same direction as the stimulating photon. This makes the two photons effectively indistinguishable in these properties and is what gives the emitted light its coherence and directionality. If the photons had different wavelengths, a random phase, or came from a different mode, it wouldn’t be a true stimulated emission event into the same mode.