What causes the color changing effect in photochromic lenses?

Photochromic lenses change color in response to UV light due to the presence of silver halide crystals embedded within the lens material. When exposed to ultraviolet radiation, these crystals undergo a photochemical reaction that alters their structure, leading to a darker tint as they absorb more light. This is a reversible process; when the UV light is no longer present, the lens gradually returns to its clear state as the crystals revert to their original configuration. The ability to transition from clear to tinted provides photochromic lenses with their distinctive functionality, making them popular in eyewear for protection against bright sunlight without the need for switching glasses.