Early attempts to image solid objects borrowed an astronomy technique called "adaptive optics," which uses algorithms to calculate how much an image has been distorted by the atmosphere. Scientists adjusted the technique to solid bodies by shining a laser through a "spatial light modulator" to delay different parts of the beam. After the light passes through the modulator and an opaque object, a detector on the other side can figure out where the scattered light came from and piece together a coherent image.
Early experiments were surprisingly successful, producing a concentrated beam a thousand times more intense than the scattered light. Inspired by the results, other teams adjusted the technique to work with focused ultrasonic waves, which shifts the laser light frequency. The shifted rays are then bounced back through the object with a mirror, adding to the energy of the initial beam and creating "a torch inside the wall" (see above). This allowed the team to image a flourescent bead just a micrometer across hidden between two opaque layers.
Someone always asks if we'll create a phone app to let people look through shower curtains... we don't intend to do that.
By speeding up the process, another team in Paris imaged the ear of a living mouse last year, a promising start for new types of body scanners. Though a lot of work is left, the technique has potential not only for medicine, but also fields like art restoration or archaeology, where experts could see what's underneath multiple layers of paint, for instance. Other shadier applications also come to mind, but as one scientist told Nature, "someone always asks if we'll create a phone app to let people look through shower curtains... we don't intend to do that."