What exactly is the use of photographic films in cameras? How are the pictures generated?

By definition:



> 
> Photographic film is a strip or sheet of transparent plastic film
>  base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The
>  sizes and other characteristics of the crystals determine the
>  sensitivity, contrast and resolution of the film.
> 
> 
> 


But how exactly is a photographic film utilized in a camera?


I know that it has a sort of a silver coating. But how does it work? I read the [Wikipedia article](https://en.wikipedia.org/wiki/Photographic_film) but found it a bit difficult to understand.




[![enter image description here](https://i.stack.imgur.com/6Izt4.jpg)](https://i.stack.imgur.com/6Izt4.jpg)


A photographic film consists of a light sensitive emulsion of fine particles (grains) of silver salts in gelatine
spread on a clear celluloid strip or a glass plate. 


The grain size is very important to photographers, as this
affects the quality of the pictures produced. 


AgBr is mainly used as the light sensitive material. 


Some AgI is used in "fast" emulsions. The film is placed in a camera. When the photograph is exposed, light from the subject enters the camera
and is focussed by the lens to give a sharp image on the film.


The light starts a photochemical reaction by
exciting a halide ion, which loses an electron.


The electron moves in a conduction band to the surface of the
grain, where it reduces a $\ce{Ag+}$
ion to metallic silver. 


[![enter image description here](https://i.stack.imgur.com/sbvkm.jpg)](https://i.stack.imgur.com/sbvkm.jpg)



> 
> $\ce{2AgBr + h\nu \rightarrow 2Ag + Br2}$
> 
> 
> 


In modern photography only a short exposure of perhaps 1/100th of a second is used. 


In this short time, only a few atoms of silver (perhaps 10-50) are produced in each grain exposed to light. 


Parts of the film which
have been exposed to the bright parts of the subject contain a lot of grains with some silver.


Parts exposed to paler parts of the subject contains a few grains with some silver, whilst parts not exposed contain none. 


Thus the film contains a latent image of the subject. 


However, the number of silver atoms produced is so small that the image is not visible to the eye.


Next the film is placed a developer solution. This is a mild reducing agent, usually containing quinol.
Its purpose is to reduce more silver halide to $\ce{Ag}$ metal.$\ce{Ag}$ is deposited mainly where there are already some $\ce{Ag}$ atoms. Thus the developing process intensifies the latent image on the film so it becomes visible. The
correct conditions for processing must be used to obtain an image of the required blackness.


[![enter image description here](https://i.stack.imgur.com/bjmZW.png)](https://i.stack.imgur.com/bjmZW.png)


[![enter image description here](https://i.stack.imgur.com/x7aMb.jpg)](https://i.stack.imgur.com/x7aMb.jpg)


[![enter image description here](https://i.stack.imgur.com/YVs4s.jpg)](https://i.stack.imgur.com/YVs4s.jpg)


If the film was brought out into daylight at this stage, the unexposed parts of the emulsion would turn black
and thus destroy the picture. 


To prevent this happening any unchanged silver halides are removed by placing
the film in a fixer solution.


A solution of sodium thiosulphate is used as fixer. It forms a soluble complex with silver halides.



> 
> $\ce{AgBr + 2Na2S2O3 \rightarrow Na3 [Ag(S2O3)2] + NaBr}$
> 
> 
> 


After fixing, the film can safely be brought out into daylight. Parts blackened by silver represent the light parts of the original picture. This is therefore a negative.


To obtain an image with light and dark the right way round, a print must be made. 
Light is passed through the negative onto a piece of paper coated with $\ce{AgBr}$ emulsion. This is then developed and fixed in the same way
as before.


**EDIT:** [Wikipedia](http://en.wikipedia.org/wiki/Photographic_film) writes:



> 
> In black-and-white photographic film there is usually one layer of
>  silver halide crystals. When the exposed silver halide grains are
>  developed, the silver halide crystals are converted to metallic
>  silver, which blocks light and appears as the black part of the film
>  negative. Color film has at least three sensitive layers,
>  incorporating different combinations of sensitizing dyes. Typically
>  the blue-sensitive layer is on top, followed by a yellow filter layer
>  to stop any remaining blue light from affecting the layers below. Next
>  come a green-and-blue sensitive layer, and a red-and-blue sensitive
>  layer, which record the green and red images respectively. During
>  development, the exposed silver halide crystals are converted to
>  metallic silver, just as with black-and-white film. But in a color
>  film, the by-products of the development reaction simultaneously
>  combine with chemicals known as color couplers that are included
>  either in the film itself or in the developer solution to form colored
>  dyes. Because the by-products are created in direct proportion to the
>  amount of exposure and development, the dye clouds formed are also in
>  proportion to the exposure and development. Following development, the
>  silver is converted back to silver halide crystals in the bleach step.
>  It is removed from the film during the process of fixing the image on
>  the film with a solution of ammonium thiosulfate or sodium thiosulfate
>  (hypo or fixer).Fixing leaves behind only the formed color dyes, which
>  combine to make up the colored visible image. Later color films, like
>  Kodacolor II, have as many as 12 emulsion layers,with upwards of 20
>  different chemicals in each layer.
> 
> 
> 


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