All I know about this is that it does but I have no idea as to why and when I ask the only answer I get is "Because it does" which isn't helpful in the least. If I could get a good, well-said response, that would be great.
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$\begingroup$ And lead to the development of photography! Silver halide crystals suspended in a gelatine film on exposure to light darkens! $\endgroup$ – Leeser Jun 15 '16 at 22:08
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The energy in the electromagnetic radiation decomposes AgCl into its components, silver and chlorine. This produces finely divided silver particles, which look dark because yet solid silver (in the form of an ingot, for example) has a typical metallic 'colour', silver powder is dark.
