An Israeli company called Deep Optics has developed sunglasses with liquid crystal lenses that can instantly be changed to reading glasses. These "adaptive focus sunglasses," as the company calls them, "allow the wearer to see at any focal point" and essentially offer "unlimited prescriptions with one pair of glasses." Check out this demo of the prototype:
As for how it works: The lower part of the lenses contain a liquid crystal layer that can change its optical power. During initial set-up, the user sets their desired magnification level—i.e. whatever their prescription for reading glasses would be—via a smartphone app. With that set, the user then only needs to swipe along the stems to trigger the magnification function. They're essentially electronic bi-focals.
The company claims that the embedded chip, battery, charging contact, Bluetooth transmitter and swipe sensors add "almost no extra weight or bulk" to the glasses. They estimate that a full charge is good for a single day's use, and point out that when the glasses aren't magnifying, they're not drawing any juice.
They're calling the sunglasses 32°N, and they're currently up on Kickstarter, starting at $229 (projected retail price: $449). At press time they had $138,389 in pledges on a $25,000 goal, with 20 days left to pledge.