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A device that generates a monochromatic and coherent beam of light. The acronym “laser” stands for “light amplification by stimulated emission of radiation”. It is based on an optical medium illuminated by a flash lamp. This causes the excitation of electrons in the optical medium. The de-excitation releases photons of the identical wavelength. The wave of photons bounces off mirrors at the ends of the optical medium and stimulates other electrons to de-excite as they pass through. A beam of monochromatic, coherent radiation is produced. Lasers have an extremely wide range of applications: surgery, laser-cutting, inertial confinement fusion, or a bar scanner in a grocery store.