Which would cause a photochromic lens to darken?

Which would cause a photochromic lens to darken?

• A. Visible light
• B. UV light
• C. Infrared
• D. All light

Answer: (B)

Photochromic lenses darken because the molecules inside the lens undergo a reversible chemical change when exposed to ultraviolet light. UV photons provide the energy needed to shift these molecules from a colorless form to a colored form, which absorbs more visible light and makes the lens appear dark. When UV exposure stops, the molecules revert to their clear state and the lens lightens again. Infrared doesn’t trigger this reaction, and while very bright visible light might cause some minor effects in some formulations, UV light is the reliable trigger for darkening.

Photochromic lenses darken because the molecules inside the lens undergo a reversible chemical change when exposed to ultraviolet light. UV photons provide the energy needed to shift these molecules from a colorless form to a colored form, which absorbs more visible light and makes the lens appear dark. When UV exposure stops, the molecules revert to their clear state and the lens lightens again. Infrared doesn’t trigger this reaction, and while very bright visible light might cause some minor effects in some formulations, UV light is the reliable trigger for darkening.