Photolysis of silver halide as an example:
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
Association within the grains produces species such as Ag2+,Ag20,Ag3+,Ag30,Ag4+,and Ag40.The free silver produced in the exposed silver halide grains constitutes what is referred to as the "latent image," which is later amplified by the development process.
The grains containing the free silver in the form of Ag4º are readily reduced by chemicals referred to as "developers" forming relatively large amounts of free silver; that deposit of free silver produces a dark area in that section of the film.The developer under the same conditions does not significantly affect the unexposed grains.