What is a Transition lens?
Transition Lenses, also known as Photochromic Lenses, are optical lenses that protect you from ultraviolet radiation by darkening when exposed to light of a sufficiently high frequency. The lenses return to being clear in the absence of activating light. Polycarbonate or another plastic material could be used to make photochromic lenses.
In response to UV light, photochromic lenses typically darken significantly in less than a minute and then gradually get a little darker over the course of fifteen minutes. Without UV light, the lenses start to clear, becoming noticeably lighter after two minutes, mostly clear after five minutes, and fully clear after about fifteen minutes.