Photon hits excited electron, makes that electron go down, and generates a new identical photon in the process, with the exact same:This is the basis of lasers.
- frequency
- polarization
- direction
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First postulated by Einstein in 1917 on his paper Zur Quantentheorie der Strahlung" ("On the Quantum Theory of Radiation") as a more elegant way to rederive Planck's law as part of the Einstein coefficients framework.
At that time there was no other physical evidence supporting the existence of the concept except that it looked more elegant.
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