Developing the first stage in film processing amplifiers the chemical changes begun by the light. It is done in the dark as the film is still light-sensitive.
In the darkroom, the film is immersed in developer, a fluid chemical mixture that reveals the image as a negative, so called because it is darkest where most light has reached the film. This is because the developer reduces the exposed silver halide grains to fine particles of metallic silver, which appear black. Before the developed film can be handled in the light, it has to be fixed – that is, unexposed silver salts are removed by immersing it in a chemical such as ‘hypo’ (sodium thiosulphate).
To make the negative into a positive print of the original scene, it is put in an enlarger and focused on silver halide coated light-sensitive printing paper. The enlarger projects the negative image on the paper at the size required, the exposes it to light. The paper retains the image in the same way as the film, but the darkest parts of the negative let through the least light, so the original light pattern is re-created. After exposure, the print is developed and fixed in a similar way to the negative.
Picture Credit : Google