You must have noticed that some of your friends wear glasses whose lenses change opacity under different lighting.Nope, they are not from Hogwartz (unlike me, haha). This is the work of Redox Chemistry.
Basically, in their lenses, called photochromic lenses, silver chloride (AgCl) and copper(I) chloride (CuCl) crystals are added during the manufacturing of the glass while it is in the molten state and these crystals become uniformly embedded in the glass as it solidifies.
One characteristic of silver chloride is its suscepitibility to oxidation and reduction by light as described below.
The chloride ions are oxidized to produce chlorine atoms and an electron. The electron is then transferred to silver ions to produce silver atoms. These atoms cluster together and block the transmittance of light, causing the lenses to darken. This process occurs almost instantaneously. As the degree of "darkening" is dependent on the intensity of the light, these photochromic lenses are quite convenient and all but eliminate the need for an extra pair of sunglasses.
The photochromic process would not be useful unless it were reversible. The presence of copper (I) chloride reverses the darkening process in the following way. When the lenses are removed from light, the following reactions occur:
The chlorine atoms formed by the exposure to light are reduced by the copper ions, preventing their escape as gaseous atoms from the matrix. The copper(I) ion is oxidized to produce copper(II) ions, which then reacts with the silver atoms as shown.
The net effect of these reactions is that the lenses become transparent again as the silver and chloride atoms are converted to their original oxidized and reduced states.
Q. Identify the oxidising agents and reducing agents in both Eq 1 and 2.
Adapted from Oxidation/Reduction by Kapi'olani Community College.
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