How sound can turn into light
Sonoluminescence occurs when intense sound waves are focused into a liquid, creating tiny gas bubbles. As the sound wave passes, these bubbles rapidly expand and then violently collapse. The temperature inside the collapsing bubble spikes to thousands of degrees Kelvin—hotter than the surface of the Sun—emitting a brief, bright flash of light.
An Accidental Glow in the Darkroom
In 1934, researchers H. Frenzel and H. Schultes at the University of Cologne were investigating underwater acoustics. While attempting to speed up the development process of photographic plates using high-frequency ultrasound, they noticed an unexpected artifact: once the plates were developed, they were covered in a dense pattern of tiny, dark speckles. Light had exposed the photographic emulsion, but the experiment had taken place entirely in a dark room. The liquid itself was emitting light in response to the acoustic field.
This phenomenon was coined sonoluminescence—literally, light generated by sound. For decades following the Cologne discovery, it remained a notoriously difficult effect to study in detail. In these early experiments, sound waves generated chaotic swarms of thousands of microscopic bubbles throughout the liquid. These bubbles moved unpredictably, coalescing, fragmenting, and collapsing in random locations, producing flashes of light that were far too faint, fleeting, and spatially dispersed for existing laboratory instruments to analyze with precision.
The Mechanics of Acoustic Cavitation
Sound traveling through a liquid is fundamentally a mechanical wave composed of alternating cycles of compression (high pressure) and rarefaction (low pressure). When an acoustic wave of sufficient intensity passes through a fluid, the low-pressure phase can tear the liquid apart or cause microscopic dissolved gas pockets to swell. This process of forming and manipulating vapor cavities in a liquid through pressure changes is known as acoustic cavitation.
During the rarefaction phase, a microbubble expands dramatically, growing to many times its resting radius as gas and vapor diffuse into the low-pressure void. However, as the acoustic cycle shifts into compression, the surrounding liquid pushes inward against the bubble wall. The surrounding fluid develops massive inward momentum. Because the inertia of the rushing liquid dominates the motion, the bubble does not merely return to its original size—it implodes violently, compressing the trapped gas into a microscopic volume in a fraction of a microsecond.