The video above shows an experiment in which a blue shining star is formed when a sound wave is passed through a bubble underwater in a flask. The phenomenon which is known as ‘Sonuluminescence’ is just outstanding.
Basically, ultrasound is used to excite a liquid which creates a larger and larger cavity (bubble) in the liquid. At a certain maximum radius, the cavity (bubble) collapses on itself (implodes) as the pressure of the surrounding liquid overcomes the acoustic field that is maintaining the cavity. The bubble collapse is powerful enough to emit a flash of light which can be seen with the naked eye. The process of bubble growth and collapse continues again and again as long as the ultrasound source is maintained.
This all happens very quickly and the bubble, when emitting the light, is tiny (about 1 micrometer). The actual time between flashes is something like a few hundred picoseconds. This is so quick that we do not notice the bubble forming and collapsing. We only see a continuous light being emitted from the liquid. It is worth noting, that the video I linked above is filmed in super-duper slow motion so that we can clearly see the mechanics as they happen.
Sonoluminescence is very interesting. There is a lot of debate on what actually causes the flash of light to be emitted. Some even speculate that the temperature of sonoluminescence is in the millions of Kelvin range... as hot as the sun.
Just look at the incredible experiment in the video above.