Sonoluminescence

In 1896 Henri Becquerel discovered that a uranium salt could darken a photographic plate, and from this effect he went on to discover radioactivity. In 1934 H. Frenzel and H. Schultes exposed a photographic plate to acoustic waves generated in a water bath and also observed a darkening of the plate. They attributed that result to luminescence from the sound field—an effect that has come to be known as sonoluminescence. The uminescence they observed did not result from the sound field directly but arose through a process called cavitation, in which voids filled with gas and vapor are generated within the liquid during the tensile portion of the pressure variation. The subsequent collapse of these voids during the compression portion of the acoustic cycle can be extremely violent and represents a remarkable degree of energy concentration—as high as 12 orders of magnitude. This energy concentration results principally from the fact that cavitation-bubble collapse obeys spherical symmetry, at least until the final stages, when instabilities in the interface may develop. This spherical symmetry is apparently preserved to submicron-size dimensions in single-bubble sonoluminescence, resulting in another remarkable phenomenon: Extremely short bursts of light are emitted from the bubble with clock-like precision.