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LIGHT SENSITIVE MATERIAL
There are two essential stages in taking photographs:
(i) Focusing an image of the object
(ii) Recording the image of the object permanently on a surface
Light is a form of energy and it can cause chemical changes in materials, it’s in fact these changes which record the image. If a material which is changed by light is exposed to light, it will change more where the image have been exposed to more brighter light and less where it have been exposed to a dim light.
Images of film and film camera spool
Some salts especially silver salts (particularly the halides e.g. silver bromide) are affected by light; these are indeed the salts used in photography to record images. A layer of these light sensitive halides is coated over a flat surface and placed where light from the image fell. By arranging the object directly on a sheet of film and then exposing it for a specific time, a negative image of the object is created in black and white. Such image created is however not permanently fixed on the film and it may be easily affected upon more exposure to light. For it to be permanently fixed, it have to further undergo additional steps in processing which will make the image resistant to any further exposure to light. This process is called development.
The art and science of photography initially used to encounter three main problems that had to be solved i.e.
(i) How to make the silver salt react to very short exposure to light e.g. fraction of a second
(ii) How to stop the image formed to resist further darkening each time it is brought to light even after development
(iii) How to turn or convert the negative image formed by silver salt to a positive image.
However, modern silver-based salts have solved these problems. These salts are able to pick light which is just brief enough to initiate the darkening process of the image (at these stages, the image is not visible to the eye). The silver-coated film is kept in total darkness and is treated with a chemical solution, which develops and accelerates the chemical change until when a strong visible black silver image appears.
These image is however not permanent and it have to be further fixed using another chemical which makes the remaining silver salts to be sensitive to any further exposure to light. These remaining silver salt can now be washed away and a stable negative image is left behind which can safely be left exposed to light without any further changes occurring. Converting the negative image into a positive image is done by shining a beam of light through the negative into another light sensitive sheet of photographic paper. This paper is then developed and fixed to give a positive image that can now be enlarged into various desired sizes and many copies made from it.