Photochromic materials are substances that change color when exposed to light, particularly ultraviolet (UV) light. These materials can transition from a clear state to a darker or tinted state, and they revert back to their original color when the light source is removed. This property makes them useful in various applications, such as in photochromic lenses for eyeglasses that darken in sunlight. The mechanism behind this color change involves a reversible chemical reaction within the material. When UV light hits the photochromic compound, it triggers a transformation in its molecular structure, leading to the color change. These materials are commonly used in sunglasses, windows, and smart textiles.