In a laser, light is produced by
- A. spontaneous emission only
- B. stimulated emission, in which a passing photon triggers an identical photon from an excited atom
- C. heating a filament
- D. the photoelectric effect
Explanation
The triggered photon matches the original in frequency, phase and direction, so the beam is coherent and highly directional, which is what distinguishes laser light from that of a lamp. Achieving it requires a population inversion, with more atoms excited than in the ground state, maintained by pumping. Metastable states are needed so that atoms remain excited long enough.
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