Photochromic materials are substances that change color or transparency in response to light exposure. When exposed to ultraviolet (UV) light, these materials darken, providing protection from harmful rays. This property is commonly found in certain types of glasses and lenses, making them useful for eyewear. When the light source is removed, photochromic materials return to their original state. This reversible process allows for convenient use in various applications, including sunglasses and optical lenses. The technology is also being explored for use in smart windows and other innovative products that adapt to changing light conditions.