Okay, you may not know Snell's law offhand, but you experience it every day. Whenever you put on a pair of glasses, or see light bend through a glass of water or a prism, this rule is in action. In short, Snell's law governs the angle by which electromagnetic radiation such as light refracts, or changes direction, as it passes from one material to another and slows down. The more the radiation slows, the more it bends and the higher the material's so-called index of refraction, described by Snell.

Until now, all known materials had a positive index of refraction. But scientists from the University of California at San Diego described in Friday's issue of Science a strange composite that has a negative index, essentially reversing Snell's law. This new mix of fiberglass and copper rings and wires (see image) is far more than a curiosity, the researchers say. It may very well lead to novel electromagnetic devices and even perfect lenses, unhindered by diffraction limits and thus capable of focusing light in unforeseen ways.

Sheldon Schultz and his colleagues produced the class of composites last year, predicting at the time that it would defy a number of ordinary properties, including the Doppler effect. Their recent demonstration is only the first of what they hope will be several to reveal the composites' unorthodox behavior. In this case, they showed that microwavesat the same frequency as those used in police radar gunsemerged from the material in the exact opposite direction from that predicted by Snell's law. Next they hope to extend the material's powers to focusing visible light.

"If these effects turn out to be possible at optical frequencies, this material would have the crazy property that a small flashlight shining on a flat slab would produce a focus at a point on the other side," Schultz says. "There's no way you can do that with just a flat sheet of ordinary material." The Defense Advanced Research Projects Agency (DARPA) and the Air Force Office for Science Research (AFOSR), which supported the research, are studying possible applications, and the U.C.S.D. researchers have filed a patent application