All About Eyes Abuja - Lenses that go dark in the sun are referred to as #photochromic lenses. They have tiny molecules embedded in them that react with UV (ultraviolet) sun rays
Eye glasses that can be manually switched to "sunglass mode" may be on their way
Acuvue Oasys Transitions: Contact lenses that get dark in the sun - CNET
The Science of Shade: Unlocking Photochromic Lenses | HowStuffWorks
Transition Brown vs Gray | Photocromic Lenses | RX Safety