Sonoluminescence occurs when sound waves are directed at a bubble in a liquid and it collapses so strongly that energy related to the collapse leads to light being emitted.
The light that is emitted is usually ultraviolet light. People have found that changing the type of gas inside the bubble can significantly change the amount of light that is emitted.
Scientists think that the process works like this. The gas in the bubble gets hotter as it collapses. The heat ionises some of the gas. In other words, the heat causes some parts of the gas to become charged. When a charged particle changes its velocity, it emits light.
(I have taken most of this information from Rev. Mod. Phys. 74, 425–484 (2002) “Single-bubble sonoluminescence” by Michael P. Brenner, Sascha Hilgenfeldt and Detlef Lohse.)
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