Which ingredient in photochromic glass darkens the lens when exposed to ultraviolet light?

The ingredient that darkens photochromic lenses when exposed to ultraviolet light is silver halides. These compounds undergo a chemical reaction when UV light is absorbed, leading to the formation of colored species which darken the lens. This property is integral to the function of photochromic lenses, allowing them to provide additional protection from bright sunlight by automatically adjusting their tint based on UV exposure.

In contrast, silver nitrate is not typically used for this purpose in photochromic glasses; rather, it is more commonly found in other applications, such as in photography. Iron oxide is generally recognized for its role in coloring and providing certain optical properties, but it does not exhibit the same photochromic behavior as silver halides. Lastly, while magnesium fluoride is often applied as an anti-reflective coating on lenses, it does not influence the darkening effect that occurs in response to UV light. Thus, silver halides are the key ingredients in creating the desired light-sensitive properties of photochromic lenses.