What characteristic of silver halide crystals allows them to function effectively in photochromic lenses?

Silver halide crystals are used in photochromic lenses because of their ability to expand when exposed to ultraviolet (UV) light, such as sunlight. This expansion causes a chemical reaction that changes the lens color, darkening it to provide protection from bright light. When the light exposure decreases, the crystals contract, and the lenses return to their clear state.

This unique characteristic is what allows photochromic lenses to automatically adjust to changing light conditions, providing convenience and comfort for the wearer. The ability to switch between clear and tinted states is a direct result of the physical changes that occur in the silver halide crystals.

Other options do not accurately describe the properties that enable photochromic lenses to function effectively. For instance, changing color in response to temperature doesn't correlate with the mechanism of photochromic adjustments. Similarly, being lightweight and flexible or moisture-resistant do not play a critical role in the operation of photochromic technology; rather, they are attributes that may apply to other aspects of lens design or materials used in eyewear.