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When this energy source gives energy the electrons in the atoms, they become excited. But the interesting part is when a few of the electrons drop back to lower energy levels spontaneously, and then release a photon (or a quantum of light). The photons created stimulate other excited electrons and emit more and more photons. Then the light waves (photons), which passes through the laser medium, is reflected by the mirrors at both ends. It keeps the light back and forth creating a chain reaction. After that, the light beam goes out through the output.
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