Photochromism is a phenomenon where a material changes color when exposed to light. This change is usually reversible, meaning the material can return to its original color when the light source is removed. Common examples of photochromic materials include certain types of sunglasses that darken in sunlight and return to clear indoors. The underlying mechanism of photochromism involves the alteration of molecular structures in response to light. When exposed to ultraviolet (UV) light, some molecules undergo a transformation that changes their absorption properties, leading to a visible color change. This property is utilized in various applications, including optical devices and smart windows.