The photographic process

A photographic film, plate, or paper consists of a support (film, glass, or paper) to which a light-sensitive emulsion is applied.
The emulsion consists of tiny crystals of a silver halide (usually silver bromide, silver chloride, or both) suspended in gelatin. When the material is exposed to light, chemical reactions within the emulsion produce an invisible “latent image.”
This image is made visible by developing the material with the now exposed light-sensitive emulsion (which contains the latent image).
When the exposed material comes in contact with the developer solution

1. the developer acts on the deposited silver halide grains
2. frees the silver from the compound
3. and deposits it as tiny, irregular grains of metallic silver in the emulsion
4. where the density of these black grains at a given location determines the density (blackness) of the image.
   The more light received by any area of the emulsion, the greater the number of silver grains present, and thus the greater the density of the image. The range from black to white in a print or negative is simply the range from a large number of developed silver grains per mm² to very few grains.
   

Developing

The developer also acts – albeit very slowly – on the unexposed silver halides in the emulsion, producing a low overall density (gray fog). Under normal developing conditions, this does not adversely affect the image, since the amount of metallic silver formed is very small.

Fixing

When development is complete, the undeveloped (unexposed) silver halide crystals remaining in the emulsion must be removed with a so-called fixer, otherwise they will darken the image. The crystals are removed from the emulsion by fixing the material in a solution of sodium or ammonium thiosulfate (“hypo”). The hypo forms a soluble compound with the unexposed and undeveloped silver halide, freeing it from the gelatin and leaving the developed silver image.

Washing

After dissolving the undeveloped silver halides, the emulsion is still saturated with the chemicals of the fixing bath and with some of the dissolved silver salts. These, if left, will slowly decompose and cause fading and discoloration of the image. To make stable prints or negatives, the hypo and silver salts must be removed by washing.