Photochromic lenses, also known as transition lenses, are a type of spectacle lens that can automatically adjust their level of darkness or tint in response to changing lighting conditions. These lenses contain special photochromic compounds that react to ultraviolet (UV) light. When exposed to UV radiation, such as sunlight, the lenses darken, providing sun protection and reducing glare. When the UV light diminishes, the lenses gradually return to their clear state.