Photochromic lenses

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. 

Combining the half reactions gives a full picture of what occurs. Metallic copper reacts in nitric acid and in so doing one copper atom is oxidized from Cu to Cu + as two nitrogen atoms are reduced from a+5 oxidation state to a +4 oxidation state. Both atoms and electrons are conserved. This balanced chemical equation shows this. 

Remember, redox reactions are only those involving atoms that change oxidation states. If none of the atoms in a reaction change oxidation state, the reaction is not a redox reaction. For example, sulfur dioxide gas, SO2, dissolves in water to form an acidic solution containing a low concentration of sulfurous acid, H2SO3. 

No atoms change oxidation states, so this is not a redox reaction. 

When a solution of sodium chloride is added to a solution of silver nitrate, an ion-exchange reaction occurs and white silver chloride precipitates. 

---

Photochromic lenses for eyewear serve as variable density optical filters. Other appHcations for photochromic light filters have been proposed including gla2ing appHcations for solar attenuation, variable transmission camera lenses, and shields for protection against the light flash from a nuclear explosion. 

The photochromic lenses contain a thin layer of a silver-containing glass, the silver being in its +1 oxidation state. Absorption of a photon supplies the energy for an electron-transfer reaction in the glass, the product of which is atomic silver ... 

Photochromic dyes, 20 516 Photochromic glass silver in, 22 658, 686 as a solar energy material, 23 5 Photochromic lenses, 6 588, 601-602 Photochromic materials, 6 587-606 inorganic, 6 589-592 organic, 6 592-601 polyoxometalates, 6 591-592 silver halide-containing glasses, 6 589-590... 

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. 

J. Crano, W. S. Kwak, and C. N. Welch, in Spirooxazines and their use in photochromic lenses, Applied Photochromic Polymer Systems (C. B. McArdle, ed.), pp. 31-79, Blackie, Glasgow and London (1992). 

Spirooxazines and Their Use in Photochromic Lenses, J. C. Crano, W. S. Kwak and... 

Most people are not aware of the optical properties of these films until they wear photochromic lenses and work in or travel in automobiles illuminated through this type of glass. This type of glass also is used to save many colored objects from fading. 

Besides classical inorganic glasses, there are certain optical plastics that are employed in the transparency and eyewear industry. For instance, thermoset resins based on allyl diglycol carbonate, poly(methyl methacrylate) derivatives, and bis-phenol A polycarbonates have been used to produce commercial plastic non-photochromic and photochromic lenses. As far as has been disclosed by the manufacturers, indolinospironaphthoxazines, INSO, and pyridobenzoxazines. 

The photochromic compounds are incorporated at a concentration of 0.1-0.3%, either by admixing or by chemical bonding. In the latter case, modified compounds with appended polymerizable functionalities are employed. Photochromic lenses operate on the basis of UV activation and thermal bleaching as shown in Fig. 5.16. 

As with most photochromic lenses, the performance of plastic photochromic lenses is temperature-dependent. In addition to variable light attenuation, photochromic lenses offer protection against UV light. Photochromic plastics coated onto classical glass lenses provide abrasion/scratch resistance and highly functional antireflectivity. For further details, the reader is referred to a review article [61]. 

Variable optical transmission materials such as photochromic camera filters [114] or photochromic lenses used in sun protection glasses [124] are shown in Figure 40.13. 

---