Page 32 - Curriculum Visions Dynamic Book

Silver in photography  These two test tubes show the effect of light on some silver
Because silver compounds are light-sensitive, about four-tenths of all the silver used industrially goes in to making photographic film.
The film is made of a plastic base over which is spread a thin layer of gelatin that contains silver salt. This gelatin layer
is known as the emulsion layer.
Various silver salts are used in the emulsion. Silver iodide reacts fastest to light
and is used for fast-speed films. Silver
bromide is slightly less sensitive and is
used for slower speeds. Silver bromide and silver chloride are light-sensitive chemicals that are placed on the surface of developing papers.
How light-sensitive silver salts work
Silver salts become reduced upon exposure to light. This converts parts of the salts to silver metal.
When the film is placed in the developer, another chemical reaction takes place, in which the silver salts are further reduced. Those
that were not affected by light are reduced more slowly, leaving some areas darker
than others.
The developed film is placed in a “fixing” solution to stop the process after it has fully reacted with the silver salts that were exposed to light. The process leaves a negative image on the acetate film.
A print is made by shining
light through the negative on to photographic paper that has
a coating containing silver
chloride crystals. This
material has to be developed
and fixed in the same way
as the film. At the end of the process a positive print is obtained.
compounds. The test tube above shows a precipitate of silver chloride the instant after it was produced. The one below shows the change that has occurred within one minute. The way that silver salts darken on exposure to light is the basis of the photographic process.
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 A film negative containing stabilised silver (the dark part)
within the gelatin. Although
it is no longer light-sensitive, it still has a gelatin layer and so is
liable to be scratched if handled carelessly. The clear areas are the
places where no light reached the film. The silver salts have been washed
away from these areas.