Which of the following choices properly describes how a laser works? a) a gain medium (lasing material) is pumped with energy to an excited state with a high-population by means of light or electrical discharge. when the excited state molecule relaxes, a photon is emitted that is can perturb another excited state molecule, causing the emission of a photon with the same energy, phase, and polarization in a process called stimulated emission. the lasing material is placed between two mirrors, one that is highly reflective and one that allows some light to leave. the stimulated emission reflects back and forth between the mirrors causing more stimulated emission, and a small fraction of light is leaked out as the functional laser light. b) coherent laser radiation is directly emitted from a gain medium that is pumped by an external energy source (light, electrical). the emitted radiation is passed through a series of filters to produce monochromatic light that is all of the same polarization. the uniformly-polarized light is then sent into a second laser gain medium, where the process of stimulated emission occurs. during this process, an excited state molecule relaxes and emits a photon that will then perturb another excited state molecule, causing it to emit a second photon. this amplifies the light, which is then sent directly out of the laser cavity. c) the lasing material is first moved to an excited state with electrical current, causing the emission of highly coherent photons of reduced energy. these photons are passed through a high-power electrical amplifier, followed by a lens system designed to create light of high power and uniform polarization. the light is then passed into a gain medium, where the process of stimulated emission occurs as a photon emitted from an excited state molecule that perturbs another excited state molecule, causing the emission of an identical photon and a population inversion. the wavelength of the output photons can be tuned using a birefringent filter, allowing the use of frequencies of a wide range.