Photochromic (“transition”) lenses automatically darken in bright sunlight and clear up again in the shade or indoors. The effect comes from silver-halide crystals inside the lens: ultraviolet light makes them rearrange into tiny clusters that absorb light, darkening the lens. When the UV light is gone, the process reverses and the lens lightens again.
One pair adapts to changing light all day — no need to swap glasses as you move between sun and shade.