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Animation showing the stimulated emission of photons, the process that gives rise to laser light. Initially an atom has electrons (blue) in the ground (lowest energy) state. A photon (yellow) from the surrounding electromagnetic field is absorbed by the atom, raising one of the electrons to a higher energy level (further from the nucleus, red). In stimulated emission, another photon is absorbed by the atom, causing it to fall back to the ground state, with the release of two photons in the same direction, with the same phase, frequency and polarisation. In a laser, this process happens repeatedly, giving rise to a thin coherent beam of light, in which every photon is in phase and has a single frequency. See K002 8522 for the mechanics of a working laser. Also see K002 8521 for an animation of the related process of spontaneous emission.
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