Photogray lenses contain small embedded crystals of solid silver chloride. Silver chloride is light-sensitive because of the reaction $$ \operatorname{AgCl}(s) \stackrel{\text { hv }}{\longrightarrow} \mathrm{Ag}(s)+\mathrm{Cl} $$ 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?

Silver chloride is sensitive to light, meaning that it undergoes a reaction when exposed to light, producing metallic silver and releasing chloride ions. This process causes the substance to darken, as small particles of metallic silver are created. The reaction can be written as: \[ \mathrm{AgCl}(s) \stackrel{\text { hv }}{\longrightarrow} \mathrm{Ag}(s) + \mathrm{Cl} \] In photogray lenses, this reaction is reversible. When the lenses are removed from light, the metallic silver particles react with chloride ions and convert back into silver chloride, causing the lenses to return to their original, transparent state. This reversibility can be explained by the presence of additional components in the lenses, which facilitate the reverse reaction and enable the lenses to return to their original state. However, when pure white silver chloride is exposed to sunlight, the reverse reaction does not occur in the dark. This suggests that the additional components in photogray lenses that facilitate the reverse reaction are absent in pure silver chloride, making the darkening effect irreversible.

Over time, photogray lenses can become permanently darkened. This permanent darkening can be attributed to the accumulation of metallic silver particles in the lenses. As the lenses are repeatedly exposed to light and dark conditions, the reversible reactions occur multiple times. However, it is possible that not all metallic silver particles revert back to silver chloride when the lenses are removed from light, causing a gradual increase in the concentration of metallic silver in the lenses. With an increasing amount of metallic silver in the lenses, the darkening effect becomes less reversible, ultimately leading to the lenses becoming permanently darkened. It is important to note that this permanent darkening is a gradual process and occurs over a long period of time due to the constant exposure of the lenses to light and dark conditions.