The Soluminescence Process.
Sonoluminescence is the conversion of sound into light. Ultrasonic waves are aimed at an air bubble in a small water cylinder. The sound waves cause the bubble to oscillate furiously: (a) the bubble starts out at a size around 5 microns (millionths of a meter); (b) then it expands to a maximum size (not to scale) of about 50 microns. At this large size there is a near-vacuum inside the bubble because of the relatively few air molecules present. This low-pressure near-vacuum region is surrounded outside the bubble by a much higher-pressure region, which causes (c) a catastrophic collapse of the bubble to between 0.1 and 1 microns. During this compression phase a flash of light (d) emerges from the bubble. (By Malcolm Tarlton, American Institute of Physics).