In laser physics, population inversion refers to which condition?

• A. The population is evenly distributed between energy levels
• B. The excitation mechanism provides energy to pump the active medium into a condition where a higher energy state is more populated than a lower energy state
• C. The population between energy levels is reversed due to cooling
• D. Population inversion occurs only at absolute zero

Answer: (B)

Population inversion means having more atoms in a higher energy level than in a lower one, which is essential for laser action because it allows stimulated emission to dominate over absorption. This state is achieved by pumping energy into the active medium, raising populations to higher energy levels faster than they decay to lower ones. That’s exactly what the described condition states: the excitation mechanism provides energy to push the active medium into a situation where a higher energy state is more populated than a lower one. The other ideas don’t fit because equal populations correspond to thermal equilibrium with no gain, cooling does not create an inverted population, and absolute zero isn’t a mechanism that produces inversion.