How do photochromic lenses respond to sunlight?

Photochromic lenses are designed to respond to sunlight by darkening when exposed to ultraviolet (UV) rays. This process occurs due to a chemical reaction that takes place within the lens material, which alters its structure and reduces the amount of light passing through the lenses. The primary purpose of this reaction is to provide enhanced protection against UV radiation, which can be harmful to the eyes. As a result, the lenses not only serve a functional purpose by reducing glare and improving visual comfort in bright conditions, but they also contribute to overall eye health.

The other options do not accurately describe the behavior of photochromic lenses. For instance, these lenses do not remain clear regardless of light levels; they specifically activate in response to UV light. They also do not change shape to reduce glare, as this is not a characteristic of photochromic technology. Finally, photochromic lenses do not tint only in response to visible light but are primarily activated by UV exposure. Thus, the correct understanding of their functionality lies in their reaction to sunlight, particularly through UV radiation.