This brief page treats some of the basic principles underlying the standard photographic process involving exposure and development of black and white film.
Black and white film uses small silver chloride crystals suspended in an emulsion, coated on a transparent backing (In the early days of photography, glass plates were used as the transparent backing). Photons striking the AgCl in a crystal grain cause ionization into Ag+ and Cl-. The greater the amount of light impinging on a grain, the more Ag+ions are produced. Those crystals that have been exposed to light have their chlorine ions removed by the "Developer", leaving behind the Ag+ now reduced to the metallic state as black silver crystals. After development the "Fixer" removes any silver chloride that was not light-activated. The small silver crystals form opaque areas on the film. This means that when one looks at the film, illuminated from behind, those parts that are exposed to light are dark (hence the term negative). The more light, the more silver gets left behind on the film, up to the maximum density of the film. To make a print, the process is repeated by shining light through the film onto a piece of white paper coated with a silver chloride emulsion, in effect reversing the above process so that white to light gray areas occur where little or no light passed through the negative and dark areas correspond to the clear areas in the negative (little activating light and hence no Ag concentration).
More information on the photographic process can be found on page 10-2.