Eyeglasses photochromic

Photochromic lenses are often used in prescription eyeglasses—eyeglasses that are worn to correct vision. Having photochromic lenses protects the eyes and, similar to sunglasses, shields the user from bright sunshine outdoors. Some prescription eyewear can be tinted like sunglasses. 

Photochromism has many potential and existing applications that take advantage of a change in colour or other physicochemical properties during the process, for example variable-transmission optical materials such as photochromic eyeglass or ophthalmic lenses that darken in sunlight (using spiropyran and spirooxazine systems in addition to... 

Photochromic eyeglasses darken when exposed to ultraviolet light and become transparent again in the absence of ultraviolet light. This process is the result of oxidation-reduction reactions. Silver chloride and copper(l) chloride are embedded in the lenses. The chloride ions absorb photons, and the silver chloride dissociates and forms chlorine atoms and silver atoms. The elemental silver darkens the lenses. Note that the chlorine ions are oxidized and the silver atoms are reduced. Then, the copper(l) ions reduce the chlorine atoms and form copper(ll) ions. In the reverse process, the copper(ll) ions oxidize the silver atoms back to the transparent silver ions. 

Naphthopyran dyes show an interesting photochromic effect. Exposure to UV light converts these colorless compounds reversibly to colored compounds, as shown below. These compounds can be used in self darkening eyeglasses. Identify the pericyclic reaction and describe the allowed stereochemistry [57]. 

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