Photography (photo + graphy) is a process of obtaining a lasting image of the object by means of electromagnetic radiation (mostly light, i.e. the visible part of the spectrum) on the photosensitive surface. The light reflected from the real object is projected onto the photosensitive surface by an optical system, usually camera lens, and causes it to change in a way which is photochemical in traditional photographic processes, and photoelectric in digital photography. The term “photography” was first used by British scientist J. F. W. Herschel in 1839.
The majority of conventional photographic processes are based on the distinctive photolytic characteristics of silver salts. Among them, the fundamental and the oldest one is the negative-positive process. It is used not only for black and white photos, but for color photographs as well, which can be made on paper or on transparent film (slides).
Negative-positive process is based on illuminating (exposing) a layer of photographic emulsion coated over a photographic film or plate. This photosensitive layer is made of very fine crystals of silver halides (mostly silver bromide). On the illuminated parts of the layer photochemical reaction occurs, disintegration (photolysis) of silver halide (reduction of silver ions into atoms) and secretion of black crumbs of elemental silver. Their density is proportional to the intensity of light, but not enough for the image to become visible. Therefore, such latent image must be subsequently made visible with chemical treatments (developing and fixing) and converted into a visible image. The resulting image is called a negative, because its dark (opaque) parts correspond to bright parts of the photographed object and vice versa. The actual image, a positive, is obtained by repeating the procedure, i.e. by contact copying or projection enlargement of a negative and its copying on photographic paper, redeveloping and fixing. The possibility of creating an unlimited number of copies of the same positive is an important advantage of this process.