What specific feature do photochromic lenses have?

Photochromic lenses are designed to automatically adjust their tint based on the presence of UV light. When exposed to sunlight or UV rays, these lenses darken to provide protection from glare and harmful UV radiation. This adjustment helps enhance visual comfort and protects the eyes in bright conditions. When the UV light source is removed, such as when indoors or during the evening, the lenses revert to their clear state. This adaptability makes photochromic lenses particularly popular for individuals who desire versatility in different lighting conditions.

The other characteristics listed—being completely clear at all times, scratch resistance, and providing a mirrored effect—do not accurately describe the unique functionality of photochromic lenses, which specifically respond to UV exposure.