(Link updated August 2004)
Ultrasound in liquids creates cavitation bubbles, which as they collapse can generate immense temperatures and pressures, and brief faint flashes of light - called sonoluminescence. It's a strange phenomenon, not yet fully understood but very reproducible. This article gives a clear account of the equipment required to generate and study sonoluminescence - a surprisingly simple set up suitable for a high-school laboratory.
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