What distinguishes photochromic lenses?

Photochromic lenses are characterized by their ability to automatically darken when exposed to ultraviolet (UV) light, which is a key aspect of their function. When individuals wear these lenses outdoors, they react to sunlight by darkening, thus providing enhanced protection against UV rays and reducing glare. This functionality adapts to changing light conditions, making photochromic lenses particularly convenient for people who move between indoor and outdoor environments.

The other choices mention features that, while they may be desirable qualities in lenses, do not specifically define photochromic lenses. Lighter weight, scratch resistance, and improved contrast are benefits that can vary among different lens types but are not unique to photochromic technology. Therefore, the defining characteristic that sets photochromic lenses apart from other lens types is their ability to adjust to UV light, providing a versatile solution for eyewear needs.