Photochromic materials are special substances that change color when exposed to light. They can darken in sunlight and return to their original color when the light is removed. This property makes them useful in various applications, such as sunglasses and photochromic lenses. These materials work by undergoing a chemical reaction triggered by ultraviolet (UV) light. When UV light hits the material, it causes a change in the molecular structure, leading to the color change. Once the light source is removed, the material reverts to its initial state, making it a practical choice for products that need to adapt to changing light conditions.