In the days of film, an ‘Exposure‘ was created on the film by chemically changing a layer of Emulsion on the film. Later, in processing, the changed chemicals could be treated so they created a picture. The Emulsion is a Microscopically thin layer of chemicals on a plastic or celluloid file. In the gelatine the light-sensitive ingredients are suspended. They are triggered by the exposure creating a chemical reaction. This results in the photographic image.