Water music by laser.

The light from a laser can generate sound by heating the medium through which it travels. A laser beam directed into water, for example, heats up the illuminated column of water, which expands slightly. If the laser's intensity oscillates, the water undergoes periodic expansions and contractions, producing an underwater sound wave.

"Laser-induced sound has been observed experimentally," says Yves H. Berthelot of the University of Texas in Austin, "but unfortunately the efficiency of this type of sound generation . . . is extremely low. We would need a laser of several thousands of megawatts to produce any decent music underwater."

Nevertheless, because this method allows underwater sound generation without actually having a physical device in the water and may lead to a new type of sonar, several researchers are now looking for ways of improving its efficiency. Recently, Berthelot showed that if a laser beam scans a water surface at the speed of sound in water, virtually all of the sound emitted by the source, though initially spread out over time, is received simultaneously. This drastically increases the sound level.