Sonoluminescence

Sound energy is converted directly into light energy by a phenomenon called sonoluminescence. Discovered in the 1800s, the process lay dormant for more than 100 years, only to experience a revival in the 1990s. How does one convert a small amount of sound energy into a brief but brilliant flash of light?

Answer

The light produced by sonoluminescence must originate in atomic transitions, electrons in excited states in atoms jumping down to lower energy levels and emitting photons to conserve energy and angular momentum. The apparatus consists of distilled water with an admixture of a little helium or other inert gas in a spherical flask surrounded by a piezoelectric crystal or two to send in sound waves at practically any frequency. The details of the apparatus can be found at many sites on the Internet.

The sound energy creates bubbles in the water that rapidly collapse and emit a flash of light from their central region. Instead of sound waves, a powerful laser pulse also can create the bubbles for the pulse of light. The spectrum of the emitted sonoluminescent light pulse is similar to a blackbody spectrum of an object at about 8,000 K, hotter than the Sun’s surface temperature of about 6000 K! And the pulse of light lasts for picoseconds, with such an intensity that it can be seen by the unaided human eye.

The reference below provides experimental results that support the popular theory that a plasma inside the bubble causes sonoluminescence. The research team fitted their pulses’ spectra to a blackbody radiation curve and found the correspondence to plasma temperatures at about 8000 K. The gas in the bubble becomes a partially ionized plasma, and the radiation is emitted by an energy cascade from ions to electrons and finally to photons.

More details will be understood eventually as faster optical response systems become available to better follow the time development of the light emission process. In fact, how quickly a state-of-the-art photo detector system operates is measured against what initial parts of the sonoluminescent pulse of light can be discerned!