What are transition lenses and how do they work to darken in the sun?

Transition lenses are a type of eyeglass lenses that automatically darken when exposed to sunlight. They contain special photochromic molecules that react to UV light, causing the lenses to darken. When the UV light diminishes, such as when indoors, the lenses gradually return to their clear state. This allows the lenses to provide both vision correction and sun protection without the need to switch between regular glasses and sunglasses.