Ultrasound makes sonoluminescent bubbles

What you're seeing is the rarely-observed phenomenon of sonoluminscence. Gas bubbles are collapsing, and part of the energy of their collapse is being translated into visible light. Here's how it happens!

A liquid is rarely entirely pure. It is usually full of salts, swirling pieces of grit, and dissolved gas. When it is agitated by something like the vibrations of sound energy, small pressure waves move through the liquid. As the pressure of some parts of the liquid decreases, the gas stops being dissolved, and forms bubbles. It's the same principle that causes an opened bottle of soda to fizz. The pressure release brings the gas bubbles out.

But a bottle of soda has a permanent release of pressure. Sound energy only temporarily takes the pressure off a certain stretch of liquid. When the pressure comes back on, the bubbles collapse with a great degree of force. This force heats the liquid near enough that sometimes the molecules of the liquid come apart, and creates temperatures comparable to the surface of the sun. A tiny bit of that energy goes into the formation of light. Most of the time, it's not visible. In this picture, the application of ultrasound to water is making a lot of sonoluminescent bubbles.

[Via NSF]