laser
n.
A device which produces an intense, usually narrow, monochromatic beam of coherent light; called also optical maser . It has various forms, and is used in various applications in science and technology. The beam is produced by boosting the majority of the electrons in specific radiation-absorbing atoms in a medium into a higher energy level, from which they are stimulated by their own emitted radiation to drop back synchronously to their lower energy level, and emit light which is in phase (coherent).
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