PhotoChromic lenses work through a chemical reaction triggered by exposure to ultraviolet light. The key component in these lenses is a PhotoChromic compound.

When exposed to UV light, like in sunlight, the PhotoChromic molecules undergo a chemical change. The UV light excites the molecules, causing them to absorb energy and transition to a different molecular configuration making your lenses darker. 

When the UV light decreases, when you go indoors or when the sunlight diminishes, the PhotoChromic molecules revert to their original configuration and your lenses gradually return to their original state