The lens of the discoloration glasses can darken after the rays of the sun. When the lighting is weakened, it becomes bright again. The reason for this is that silver halide crystals are working. Under normal circumstances, it can make the lens to maintain perfect transparency. In the case of sunlight, the silver in the crystal is separated, and the free silver forms a small aggregate inside the lens. These small silver aggregates are irregularly staggered, they can not transmit light, but only absorb the light, the result is to make the lens darker. In the case of light and dark, the crystal is re-formed, and the lens returns to the bright state.