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, tinted state, and then revert back to clear when the light source is removed. This property is commonly used in products like photochromic lenses for glasses, which darken in sunlight and clear up indoors. The mechanism behind photochromism involves chemical compounds that undergo structural changes when they absorb UV light. These changes alter the way the material interacts with visible light, resulting in the color change. Photochromic materials are also found in applications such as smart windows and sensors, enhancing functionality and user experience.