Sonoluminescence
If you look very closely at the the above image you’ll see a tiny pin point of light in the round bottom flask. While it may not look like much, the physics behind this flash of light are as amazing as they are mysterious.
The process of sonoluminescence is an interesting one, as the name suggests it’s light that is produced via sound. The principle behind it is that a liquid when exposed to a strong enough “sound wave” at the right frequency will produce bubbles through cavitation. Cavitation in turn is when water is displaced but there is nothing to replace it, think of moving your arm very fast underwater and seeing that empty pocket behind it. By setting up a standing wave in the liquid using pulses of sound bubbles are produced (although sonoluminescence will work if they are already present) which have incredibly short life times. The interesting thing about these bubbles is that when they collapse they emit light and reach temperatures of 20,000 Kelvin (for reference the Sun’s surface is at a mere 5,700 Kelvin) producing glowing plasma in the process. These flashes last only a few hundred picoseconds and the bubbles themselves are only about 10 micrometers in diameter when they flash. As yet there is no perfect answer as to why this occurs.
