The light-sensitive chemicals used in most common photographic film and paper are silver halides.
A silver halide (or silver salt) is one of the compounds formed between silver and one of the halogens – silver bromide (AgBr), chloride (AgCl), iodide (AgI), and three forms of silver fluorides. Collectively referred to as the silver halides, they are often given the pseudo-chemical notation AgX.
In photographic film and paper, silver halide crystals in gelatin are coated on to a film or paper base substrate. The gelatin is a vital part of the emulsion as the protective colloid of appropriate physical and chemical properties.
When a silver halide crystal is exposed to light, some the surface of the crystal is turned into metallic silver. If the metallic silver contains enough exposed atoms, it is rendered developable – meaning that it can undergo chemical development which turns the entire crystal into metallic silver. Areas of the emulsion receiving larger amounts of light undergo the greatest development and therefore result in the highest density.
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, except in the case of some historical processes such as the collodion wet plate and daguerreotype, in which the iodide is sometimes used alone. Silver fluoride is not used in photography.