What is a laser?

Spontaneous emission is a quantum mechanical effect and a direct physical manifestation of Heisenberg's uncertainty principle. The emitted photons have random directions, but their wavelengths match the absorption wavelengths of the transitions. This is the mechanism of fluorescence and thermal emission.
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Absorption of photons with the correct wavelength by the transition can also cause electrons to drop from higher to lower levels, emitting new photons. The emitted photons are perfectly matched to the original photons in wavelength, phase and direction. This process is called stimulated emission.