In 1880 Alexander Graham Bell discovered that shining light on an elastic material elicits sound from it. His discovery, known as the photoacoustic effect, has today matured into a sensitive medical imaging technique: The absorption of visible or near-IR light pulses by molecules under the skin increases their temperature and launches a pressure wave that propagates back to the surface as ultrasound, which is picked up by an array of microphones. (See the article by Stanislav Y. Emelianov, Pai-Chi Li, and Matthew O’Donnell, Physics Today, May 2009, page 34.)
Although the photons can be focused only through a millimeter or so of soft tissue, they penetrate far more deeply—up to a few centimeters—diffusing as they scatter countless times. The photons lose their spatial memory during so deep a random walk, but the local conversion of their energy into acoustic waves restores it. The mean free path of ultrasound...
