Abstract
Abstract A simple and most instructive experiment, shown in many lectures for beginners in physics, is the demonstration of resonant absorption and resonant fluorescence (also called resonant scattering) with yellow sodium light. A beam of light produced by a lamp, in which a glow discharge is maintained in sodium vapour, is directed through an evacuated glass globe containing some metallic sodium. The beam falls on a screen behind the globe, producing a bright yellow patch. As the globe is heated and thus fills with sodium vapour, the yellow patch loses intensity and at the same time the incident beam becomes visible as a yellow glow within the glass globe. Excited sodium atoms in the lamp perform a transition to the ground state by emitting light quanta of energy E 0. These, in turn, can excite atoms in the globe and thus be absorbed from the beam. The excited atoms return to their ground state, again by emitting quanta of the energy E 0 and causing a visible glow where the incident light beam passes.
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