Sunglasses with mirrored lenses or photochromatic lenses

[bifocal glasses]

You've seen sunglasses with a mirror finish on the outside of the lenses. They're popular, but mirrored lenses scratch easily because the mirror finish is applied last. Photochromatic (or photochromic) lenses become darker when exposed to UV radiation. The shift happens quickly as your surroundings change from bright to dim.