Photochromic chemicals are substances that change color when exposed to light. They can switch between two forms, one of which is colored and the other is colorless. This property allows them to be used in various applications, such as in photochromic lenses that darken in sunlight and return to clear indoors. These chemicals are often found in materials like sunglasses and smart windows. When light hits the photochromic molecules, they undergo a chemical reaction that alters their structure, leading to the color change. This unique feature makes them useful for enhancing comfort and protecting against harmful UV rays.