Silver halides are photosensitive matierials, this means they have an ability to give up an electron, and with great ease, especially Silver Iodide, because it has the lowest oxidising power
Photography started in 1839, when Fox Talbot demonstrated the photosensitive characteristics of Silver halides, to make an image. Photography essentially relies upon redox reactions (oxidation reducion). Because the three different silver halides have different absorbtions, it meant that images of different shades (then colours in the future) could be obtained, and as soon as that was realised, the photograph was now possible.
Here is a basic description of the methods used in photography.
Small crsytals of silver halide, are suspended with a suitable support, (film) to form a photosensitive emulsion. Prolonged exposure causes darkening of the emulsion.
The support is exposed to light . A few of the silver ions in those crystals struck by light are reduced to silver atoms. Short exposures produce a so called latent image. This is called so because, the number of ions reduced directly by the light is so few that you cannot see a change in the exposed part of the print with the naked eye before the paper is developed.
2 AgBr(s) + C6H4O2(aq) 2- => 2 Ag(s) + 2 Br-(aq) + C6H4O2(aq)
Above is the basic chemical reaction for developing a film. Colour photography uses dyes mixed with silver halides, three layers of emulsion are used, incorporating the the dyes, and each sensitive to a different part of the spectral region. (red, blue, green) This results in different latent images in each layer.
The final products are then washed in sodium thiosulfate solution to remove the photosensitive Silver halides, and the image is preserved.
Graham Ross Macleod april 2003