How do photochromic glasses work?

[Seffner]

I am looking for a pair of sunglasses and when I search online I find a new type: phtochromic(maybe it's not new, but I've never heart of it) it seems that the lenses will change into different color when outdoors and indoors. How miraculous it is! How do photochromic glasses work?

[Segno]

Photochromic lenses are lenses that darken on exposure to ultraviolet (UV) radiation. Once the UV is removed (for example by walking indoors), the lenses will gradually return to their clear state.Photochromic lenses may be made of glass, polycarbonate, or another plastic.Modern photochromic lenses tend to be plastic and instead of silver chemicals they contain organic (carbon-based) molecules that react to light in a slightly different way. Instead of clumping together, they change their color and size when ultraviolet light strikes them.Although each molecule changes by only a fraction of a nanometer, when many molecules respond in the same way the effect can be more dramatic. Imagine lots of molecules suddenly darkening and fattening up inside a clear lens.

[Seguin]

Personally, photochromic glasses is really a wonder thing. They can adjust themselves automatically with the change of UV rays in the air. When you stay in room, the eyeglasses appear clear lenses as regular prescription eyeglasses for vision aids. But when you wear it in sun, the lenses can adjust themselves into black or brown colored lenses(the color of the lenses depend on which color you choose when you buy). It can not only help reduce the strong light, thus give you comfortable vision, but also can prevent UV rays, even glares in the air, therefore to protect your eyes. And all of this miraculous adjustment themselves depends on organic photochromic molecules (for example oxazines and naphthopyrans). So, I think photochromic glasses is worthy to try especially in summer.