Question

How is photolytic decomposition used in photography

Answer 1

AgBr quickly darkens on exposure to light by disintegrating into elemental bromine and metallic silver. This reaction is used in photography and film. In black-and-white photographic film there is usually one layer of silver salts. When the exposed grains are developed, the silver salts are converted to metallic silver, which blocks light and appears as the black part of the film negative.
As a photographic film is subjected to an image, photons incident on the grain produce electrons which interact to yield silver metal. More photons hitting a particular grain will produce a larger concentration of silver atoms. The film now has a concentration gradient of silver atom specks based upon varying intensity light across its area, producing an invisible "latent image".
While this process is occurring, bromine atoms are being produced at the surface of the crystal. To collect the bromine, a layer on top of the emulsion, called a sensitizer, acts as a bromine acceptor.
During film development the latent image is intensified by addition of a chemical, typically hydroquinone, that selectivity reduces those grains which contain atoms of silver. The process, which is sensitive to temperature and concentration, will completely reduce grains to silver metal, intensifying the latent image on the order of 1010 to 1011. This step demonstrates the advantage and superiority of silver halides over other systems: the latent image, which takes only milliseconds to form and is invisible, is sufficient to produce a full image from it.
After development, the film is "fixed," during which the remaining silver salts are removed to prevent further reduction, leaving the "negative" of the film. The agent used is sodium thiosulphate. An indefinite number of positive prints can be generated from the negative by passing light through it and undergoing the same steps outlined above.