​​How do Photochromatic Lenses work?

Photochromic lenses are eyeglass lenses that are clear (or nearly clear) indoors and darken automatically when exposed to sunlight.
Modern photochromic lenses tend to be plastic and instead of silver chemicals they contain organic (carbon-based) molecules called naphthopyrans that react to light in a slightly different way: they subtly change their molecular structure when ultraviolet light strikes them. In this altered form, they soak up more ordinary light as it tries to pass by (technically, we say they have a different absorption spectrum), which is what makes the lenses darken. Imagine lots of molecules suddenly darkening inside a clear lens. It’s a bit like closing the blinds in front of your window on a sunny day: as the slats turn, they progressively block out more and more light.