When creating a photographic medium such a photographic paper you generally use a carrier such as gelatin and mix in Silver Nitrate and Potassium Bromide (Silver Bromide, AgBr) to produce a light sensitive coating containing silver halide grains. The coating is applied to paper producing a light sensitive photographic material that can be used to capture an image. carried around in a dark film holder so that it can be exposed to light when the photographer wishes to do so.
When the photographic paper is exposed to light the silver halide grains become tagged, for want of a better expression. So the image is still on the photographic paper but you can't see it. In other words the image hasn't yet been formed but can be developed from the possibility of an image into something that we recognize as a physical picture.
The development uses a solution of chemicals that are able to target the tagged silver halide grains and convert only those silver halide grains that have been exposed to light to metallic silver. The silver halide grains that have been converted to metallic silver are no longer light sensitive. Any silver halide grain that hasn't received any light and hence isn't tagged remains as silver bromide and is still light sensitive. Therefore to ensure permanence of the picture all remaining silver bromide must be removed.
The removal of silver bromide is carried out using a solution called a fixer. The fixer has no effect on metallic silver but will dissolve silver halide grains and remove them, leaving only the permanent metallic silver grains, coloured black, behind. This is a negative image.
Where light falls on a normal white surface would cause the light be reflected back as a white area and where there is no light there is no reflection of light and so that area remains dark. In this way we can see a positive image when we view the ground glass on a large format camera. However in photography the opposite is true, in other words where light falls the silver halide grains are converted to metallic silver, coloured black.
To get a positive image you can use the negative to block light on another sheet of photographic paper, where there is metallic silver and where there is none light is let through, accurately reversing the negative into a positive.
Neat eh? It's genuine magic. The picture included in this post was created on photographic paper using the chemical interactions as described above. It's a thing of beauty (Or at least I think so) created by nothing more than light and chemical targetting.
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