What feature does a photochromic lens provide?

Photochromic lenses are designed to automatically adjust their tint in response to the intensity of UV light. When exposed to sunlight, these lenses darken, providing protection from glare and harmful UV rays, and they return to a clear state indoors or in low-light conditions. This ability to change from clear to tinted according to lighting conditions is precisely what differentiates photochromic lenses, making them convenient for wearers who transition between indoor and outdoor environments without needing to switch out glasses.

The other features listed do not accurately describe photochromic lenses. A permanent tint would imply a fixed level of coloration, which does not allow for adaptation to light conditions. Enhanced scratch resistance is a property that may apply to some lenses, but it is not the defining characteristic of photochromic lenses specifically, nor does it pertain to their primary purpose. Additionally, photochromic lenses are not designed exclusively for reading; they can be used for various activities, providing overall comfort and protection from light across different settings.