Silver Halide
The light-sensitive compounds — silver bromide, chloride and iodide — that form the active ingredient of photographic emulsions.
Formula / rule
AgBr, AgCl, AgI
In depth
When a photon strikes a halide crystal it frees an electron, which reduces a few silver ions to metallic silver at a sensitivity speck. These specks are far too few to be visible, but they act as catalysts during development, where the entire crystal is converted. The amplification factor is enormous — around a billion to one.
Examples in practice
A handful of silver atoms formed by light triggers the development of a crystal containing billions of them.