According to an item on NPR yesterday, a group at University of Arizona has developed a new kind of bifocal glasses. Just a prototype for now, the glasses use a layer of liquid crystal that changes optical properties in response to an electric charge:
The liquid crystal is transparent, and its molecules normally point in one direction. But they all shift if they're hit with a tiny pulse of electricity, and that changes how light passes through them. So if someone is wearing these glasses and wants to read a book, all they have to do is flip a switch. "If you put the voltage on it, it acts like a lens," Peyghambarian says. "If you take it off, it just doesn't have any power, it becomes a piece of glass."
They're working on making them a practical size, and on adding an automatic mechanism to make the switch based on what the wearer is looking at — much like the autofocus mechanisms on digital cameras. The work is described in the Proceedings of the National Academy of Sciences, but the paper doesn't appear to be on their web site yet.
This is very cool!