The active medium of a laser must be which of the following?

• A. Solid-state crystals only
• B. Gases only
• C. Solid, Liquid, or Gas and must have an excited unstable state
• D. Liquid crystals only

Answer: (C)

The key idea is that lasing relies on stimulated emission from a population-inverted medium. The active medium can be solid, liquid, or gas, as long as its atoms or molecules have discrete energy levels that can be pumped up to a higher, excited state and kept more populated there than in the lower state. That population inversion—an excited state that can decay and emit photons when stimulated—is what sustains light amplification. The excited state is not the ground state and typically has a finite lifetime, so continuous pumping is used to maintain the inversion. This is why solid-state lasers, gas lasers, and dye lasers all work, while a form like liquid crystals isn’t the typical active medium for standard laser operation.