Photochromic lenses are glasses lenses that react to different levels of UV light. When exposed to bright light on a sunny day, molecules in the lens react and become darker, offering protection from the sun’s radiation. As light levels decrease, the molecules go back to their original state and become transparent once again.
Polarized lenses are used in sunglasses to reduce glare from reflective surfaces such as the surface of a lake or the hood of a car. They accomplish this feat through a process called polarization, much like a venetian blind controls sunlight through a window.