What process occurs after population inversion has been achieved in a medium?

• A. Emission Absorption
• B. Absorbed Emission
• C. Spontaneous Emission
• D. Stimulated Emission

Answer: (D)

When population inversion is achieved in a medium, the process that follows is stimulated emission. In the context of lasers, population inversion means that there are more atoms in an excited state than in the lower energy state, thus creating a condition where emitted photons can trigger further emissions.

In stimulated emission, when a photon of a specific energy interacts with an excited atom, it can stimulate that atom to release a second photon that is coherent with the first. This means that both photons will have the same phase, frequency, direction, and polarization. This process is fundamental to the operation of lasers, as it leads to the amplification of light — a key feature of laser technology.

The significance of stimulated emission lies in its ability to generate a highly coherent and monochromatic beam of light, which is what makes lasers distinct from other light sources. The process relies on the presence of a population inversion, as a higher density of excited atoms increases the probability of photon interactions leading to more stimulated emissions.