Eyes lose their flexibility with age, sometimes making it difficult to shift focus from near to far or vice versa. To combat the problem, Benjamin Franklin devised bifocals--eyeglass lenses shaped for near viewing in the lower half and distance vision in the upper portion--more than 200 years ago. Now researchers have created liquid crystal lenses that can change between long-distance and reading modes with the flick of a switch.

Guoqiang Li of the University of Arizona and his colleagues sandwiched a thin layer of liquid crystal between two layers of glass and laced it with concentric rings of electrodes. When turned on, the electrodes reconfigure the focusing power of the lens for either near or far vision, allowing the entire lens to promote the desired effect in less than a second.

In tests on both human and mechanical subjects, the liquid crystal lens delivered a sharp image whether focusing on the close at hand or the distant. And in an improvement on earlier efforts with liquid crystals, if the electrical current fails for any reason, the lens simply reverts to its distance-viewing state. Because most people requiring bifocals have difficulty seeing up close rather than far away, this feature makes the glasses safe for activities like driving, the scientists argue. The research appears online this week in Proceedings of the National Academies of Science.

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