A standing wave of sound creates a bubble in liquid. When the bubble implodes, light is emitted. This is called: Sonoluminescence. Awesome.

Sonoluminescence is the emission of short bursts of light from imploding bubbles in a liquid when excited by sound.

In 1989 a major experimental advance was introduced by Felipe Gaitan and Lawrence Crum, who produced stable single-bubble sonoluminescence (SBSL). In SBSL, a single bubble trapped in an acoustic standing wave, emits a pulse of light with each compression of the bubble within the standing wave. This technique allowed a more systematic study of the phenomenon, because it isolated the complex effects into one stable, predictable bubble. It was realized that the temperature inside the bubble was hot enough to melt steel. Interest in sonoluminescence was renewed when an inner temperature of such a bubble well above one million kelvins was postulated. This temperature is thus far not conclusively proven, though recent experiments conducted by the University of Illinois at Urbana-Champaign indicate temperatures around 20,000 K.

:sonoluminescence:

Sonoluminescence can occur when a sound wave of sufficient intensity induces a gaseous cavity within a liquid to collapse quickly. This cavity may take the form of a pre-existing bubble, or may be generated through a process known as cavitation. Sonoluminescence in the laboratory can be made to be stable, so that a single bubble will expand and collapse over and over again in a periodic fashion, emitting a burst of light each time it collapses. For this to occur, a standing acoustic wave is set up within a liquid, and the bubble will sit at a pressure anti-node of the standing wave. The frequencies of resonance depend on the shape and size of the container in which the bubble is contained.

~ The light flashes from the bubbles are extremely short—between 35 and a few hundred picoseconds long—with peak intensities of the order of 1–10 mW.

~ The bubbles are very small when they emit the light—about 1 micrometre in diameter—depending on the ambient fluid (e.g., water) and the gas content of the bubble (e.g., atmospheric air).

~ Single-bubble sonoluminescence pulses can have very stable periods and positions. In fact, the frequency of light flashes can be more stable than the rated frequency stability of the oscillator making the sound waves driving them. However, the stability analyses of the bubble show that the bubble itself undergoes significant geometric instabilities, due to, for example, the Bjerknes forces and Rayleigh–Taylor instabilities.

~ The addition of a small amount of noble gas (such as helium, argon, or xenon) to the gas in the bubble increases the intensity of the emitted light.