Silver halide
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A silver halide is one of the compounds formed between silver and one of the halogens, usually silver bromide (AgBr), silver chloride (AgCl) and silver iodide (AgI). As a group, they are often referred to as the silver halides, and are often given the pseudo-chemical notation AgX. Although most silver halides involve silver atoms with oxidation states of +1 (Ag+), silver halides in which the silver atoms have oxidation states of +2 (Ag2+) are known, of which silver(II) fluoride is the only known stable one.
Silver halides are used in photographic film and photographic paper where an emulsion of silver halide crystals in gelatin is coated on to a film base, glass or paper substrate. The gelatin is a vital part of the emulsion as it contains trace elements which increase the light sensitivity of the emulsion. Silver bromide and silver chloride may be used separately or combined, depending on the sensitivity and tonal qualities desired in the product. Silver iodide is always combined with silver bromide or silver chloride.
Silver halides are also used to make some corrective lenses darken when exposed to ultraviolet light (see photochromism).
When a silver halide crystal is exposed to light, a sensitivity speck on the surface of the crystal is turned into a small speck of metallic silver (these comprise the invisible or latent image). If the speck of silver contains approximately four or more atoms, corresponding to an absorption of four or more photons, it is rendered developable - meaning that it can undergo development which turns the entire crystal into metallic silver.
