What is a transition lens? Transition is actually the brand name of the main provider of photochromic or "adaptive" lenses which darken when exposed to UV light (such as when you walk outdoors during daylight hours).
When you are no longer exposed to the effects of UV, the lenses return to their clear state.
Photochromic lenses should block 100% of harmful UVA and UVB rays but its worth checking with your optician to ensure that is the case. Transition lenses do block 100% of both, which is why we recommend them.