Film Photography is a strip or sheet of transparent plastic coated on one side with a gelatin containing small light-sensitive silver crystals. The sizes of the crystals determine the sensitivity, resolution of the film.
The film will darken if left exposed to light, a short exposure to the image formed by a camera lens is used to produce a chemical change, equal to the amount of light taken in by each crystal. This then creates the photograph that we see.
When black-and-white photographic film is being used there is only one layer of silver salts. When the crystals get exposed and see the chemicals and light the silver salts are converted to metallic silver, which blocks light and appears as the black part of the film which is called a negative.
Colour film uses at least three layers to create the finished photograph. one of the first steps is dyes, which absorb the silver salts and make the crystals sensitive to different colours. Following development, the silver is converted back to silver salts in the bleach step. It is removed from the film in the fix step which means all the colours are finally fixed in to place to they don't smudge or discolour.
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