What type of glass contains silver halide crystals that darken in sunlight?

Photochromic glass is a type of lens that contains silver halide crystals, which undergo a chemical reaction when exposed to ultraviolet (UV) light from the sun. This reaction causes the crystals to darken, resulting in a tint that helps reduce glare and protect the eyes from bright sunlight. When the light source diminishes or is removed, the lens gradually returns to its clear state.

This property makes photochromic lenses particularly popular for those who frequently transition between indoor and outdoor environments, offering convenience by eliminating the need to switch between prescription glasses and sunglasses. The other types of lenses listed—high-index glass, plastic lenses, and polycarbonate glass—do not have the same light-reactive properties and remain uniform in color regardless of lighting conditions. Hence, they do not provide the dynamic adjustment to light that photochromic lenses do.