A lens that adapts
Photochromic molecules react to ultraviolet radiation: outdoors the lens darkens within seconds, and clears again when you come back in. You no longer need to swap glasses on your way through the door.
When they really pay off
They suit anyone who moves in and out of doors throughout the day. Worth knowing: a windscreen filters out most UV, so the reaction is weaker behind the wheel — there are dedicated versions for driving.