Photochromic lenses, often called transitions or reactolights, darken to a sunglasses tint when exposed to sunlight, or U/V ultraviolet, and return to a clear state when indoors, away from U/V light.
Photochromic lenses are made of many lens materials including plastic, glass or polycarbonate. They are typically used as sunglasses that conveniently switch from a clear lens indoors, to a sunglasses depth tint when outdoors, and vice versa.
They typically take around 30 seconds to fully darken and approx 2 - 5 minutes to return to a clear state.
Photochromic lenses were created in the 1960's at the Corning Glass Works Inc, and became the first variable tint/photochromic lenses brought to the marketplace.
Tiny molecules of silver halide and chloride are embedded within a photochromic lens which is invisible and clear until exposed to sunlight/UV rays. A chemical process takes place when exposed to sunlight/UV and the molecules effectively move, change shape and absorb the light. This process changes the molecules and they darken to a sunglasses shade/tint.
The photochromic lens will remain in this 'darkened' sunglasses state whilst exposed to sunlight/UV and the molecules will reverse the process once the lenses are removed from the sunlight/UV rays returning to the original 'transparent/clear' lens.