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How does a laser work?

A laser is one of the few really important applications of quantum mechanics, but like quantum mechanics itself, it is a little hard to understand properly. What a laser has to do is to "invert" the "population" of atoms from it is usual distribution where most of the atoms are in very low energy electronic states. A pump adds energy to put most of the atoms in an excited state, and then a few of the atoms randomly decay, emitting radiation. This decay radiation actually increases the chance that other atoms will decay, and the others decay in synchrony with this radiation, adding to it in intensity while maintaining the phase. By placing mirrors at either end of the region where these atoms are, you can cause the radiation to bounce back and forth many times, greatly multiplying the intensity and directionality.

Arthur Smith

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