Sun Reactive, or Photo Chromatic Lenses, offer a clear (or nearly clear) lens indoors then darken automatically when exposed to sunlight. How do the lenses change color? The molecules in the lenses darken when they are activated by the sun’s ultraviolet radiation. Because UV rays penetrate clouds, the Sun Reactive lenses will darken on overcast days as well as sunny days. Sun reactive lenses will not typically darken inside a car, as the windshield and windows have been treated to block UV rays.