Chemistry of Photography
The principle of photography is based on the photochemical reaction of light on certain silver halides. The photographic technique involves the following steps: 1) Preparation of sensitive plate or film :- It is an emulsion of AgBr in gelatin uniformly coated on a glass plate or a celluloid sheet (film) in a dark room.The process is done in dark because AgBr is highly light sensitive. 2) Exposure :- The sensitive plate or film is mounted on a camera and exposed for a few seconds to the image of a properly focused object. An invisible change occurs in that part of the emulsion on which light falls.The chemical reaction is that light reduces AgBr to Ag. Br- + photon -------> electron + Br Ag+ + electron -------> Ag An image of the object is formed on the film which is not visible and therefore called latent image. 3) Development of the latent image :- To make the latent image visible, the film is passed through a reducing bath containing hydroquinone (an organic reducing agent). ...