My apprentice optician asked me how photochromic/ transition lenses so I thought I'll share my knowledge here with those apprentices or newbies.

Firstly UV coating should not be confused with photochromic treatment. Photochromic lenses are the type that darkens gradually when exposed to sunlight and return to normal when you go back indoors. The light-sensitive molecules inside photochromic lenses shift their structure when they're bathed in UV rays, allowing them to absorb more light and give the lenses a darkened appearance. Once the UV light goes away, the process reverses and the lenses become clear again.