This document discusses the principles of lasers, including: 1) Lasers work using the principle of stimulated emission of radiation, where atoms or molecules in an excited state emit photons when stimulated by an external source, producing an intense beam of coherent and monochromatic light. 2) Key terms include absorption, emission, population inversion, and stimulated emission, which are required for lasers to function. 3) Lasers have characteristics like monochromaticity, coherence, and collimation that make them useful surgical tools, though they also have disadvantages like cost and safety hazards that require special training.