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Resonance luminescence of sodium vapour
The lamp (N) emits photons with a wavelenght of 589,3 nm, which hit the sodium atoms contained in the sphere ( R). These atoms are initially in the ground state. The light emitted by the lamp is of precisely the wavelength required for the excitation of the atoms. The absorb the energy, momentarily reach a higher energy level and then leave this level, again emitting photons.
The process can be compared with two tuning forks vibration in resonance.