Color-changing lenses darken when the sun shines. When the lighting fades, it becomes bright again. This is possible because the silver halide crystals are at work.
Under normal conditions, it keeps lenses perfectly transparent. When exposed to sunlight, the silver in the crystal is separated, and the free silver forms small aggregates inside the lens. These small silver aggregates are irregular, interlocking clumps that cannot transmit light but absorb it, darkening the lens as a result. When the light is low, the crystal reforms and the lens returns to its bright state.