Photogray lenses contain small embedded crystals of solid silver chloride. Silver chloride is light-sensitive because of the reaction $$ \mathrm{AgCl}(s) \stackrel{\mathrm{hv}}{\longrightarrow} \mathrm{Ag}(s)+\mathrm{Cl} $$ Small particles of metallic silver cause the lenses to darken. In the lenses this process is reversible. When the light is removed, the reverse reaction occurs. However, when pure white silver chloride is exposed to sunlight it darkens; the reverse reaction does not occur in the dark. a. How do you explain this difference? b. Photogray lenses do become permanently dark in time. How do you account for this?

The reaction given shows that silver chloride (AgCl) decomposes to silver (Ag) and chloride (Cl) when exposed to light. The silver particles are responsible for the darkening of the lenses. This reaction is reversible, which means that the lenses can return to their initial state when they're not exposed to light.

The primary difference between photogray lenses and pure white silver chloride crystals lies in their structure and composition. In photogray lenses, the AgCl crystals are embedded within a polymer matrix, which serves as a medium for the reactions to occur. a. In photogray lenses, the polymer matrix allows for the movement and interaction of Cl with the Ag formed during the reaction. When the light is removed, Cl can interact with Ag and re-form AgCl, thus making the process reversible. On the contrary, pure white silver chloride crystals lack the polymer matrix. When AgCl is exposed to sunlight, it decomposes to Ag but doesn't have a suitable medium for the reverse reaction to occur in the dark, making the darkening process irreversible.

b. Over time, the continuous exposure to light and dark cycles causes the AgCl crystals in the photogray lenses to undergo multiple reversible reactions. However, the efficiency of these reactions might decrease due to the gradual changes in the polymer matrix or the migration of Ag and Cl ions within the lenses, which might cause the reaction to become more difficult to reverse. Additionally, the constant exposure to light can cause the degradation of the polymer matrix, affecting the reversibility of the reaction. Thus, photogray lenses may become permanently dark over an extended period.