Question

How is photolythic decomposition used in photography, also write the reaction involved?

Answer 1

Photodissociation, photolysis, or photodecomposition is a chemical reaction in which a chemical compound is broken down by photons. It is defined as the interaction of one or more photons with one target molecule.
A decomposition reaction occurs when one reactant breaks down into 2 or more products.
General equation AB--->A+B

Decomposition reaction Used In Black and White photography

Silver chloride when kept in the sun decomposes to form silver and chlorine gas.The white silver chloride powder turns grey due to the formation of silver and with the release of chlorine gas.

Similarly, silver bromide also undergoes decomposition to give silver with the release of bromine gas when exposed to air.

Since the reactions are sensitive to light they are used in black and white photography.

Chemistry Behind Black and White Photography

Black and white photography brings out details and contrast. The way black and white photographs are created depends on the exposure of light sensitive material which forms the image, in terms of film photography. It starts with silver halides which is the light sensitive material, known as AgBr. AgBr is exposed to form the image depending on the frequency of light which is involved in this process. After being exposed to light, the AgBr forms an image on the film. When developing the image, the exposed AgBr grains convert to visible metallic deposit using an oxidized reducing agent, sodium sulphate.

Explanation
In plain terms, in black and white photography, acid-base reactions occur. Light reacts with silver halide crystals to form an image. Electrons are kicked out when the photon energy from the light excites particles. This happens while in contact with the emulsion on the paper. A developing solution magnifies this reaction when the paper is placed in it . As a result, the silver sticks together and darkens in some places, forming the image. This has to be done in a dark room since previously exposed paper will turn all black.
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