To be able to understand chemistry of silver-based photography, one must understand the chemistry of silver salts. A common photographic film contains tiny crystals of slightly soluble silver halide salts such as silver bromide (AgBr) typically referred to as "grains." These grains are suspended in a gelatin matrix in a thin layer on the paper and the resulting gelatin dispersion referred to as an "emulsion," is melted and applied as a thin coating on a polymer base or, as in older applications, on a glass plate.
Below is the chemical representation of the silver halide process.
When light or radiation of appropriate wavelength strikes one of the silver halide crystals, a series of reactions begins that produces a small amount of free silver in the grain. Initially, a free bromine
atom is produced when the bromide ion absorbs the photon of light:
Ag+Br- (crystal) + hv (radiation) ___ Ag+ + Br + e-
The silver ion can then combine with the electron to produce a silver atom.
Ag+ + e- ___Ag0
The free silver produced in the exposed silver halide grains makes up
what is referred to as the "latent image," which is later amplified by
the development process.