Black and white photography -- effect of exposure to light
The main idea behind black and white photography is that white silver halides (AgCl and/or AgBr, sometimes also some AgI is present) are reduced to metallic silver. The finely divided silver particles have a black appearance and the image is created by varying the amount of reduced silver halide.
Schematically, the chemistry behind the silver halide process is like this:
AgX + red → intermediate stages → Ag + X- + red+
Here, X is one of the halogens, and red is some reducing species, which can easily pass an electron to something else, which finally ends up on a silver atom, giving metallic silver.
However, simply reducing a silver halide does not result in an image. Some selective mechanism must be introduced. In the 19th century, it was discovered that silver halides are light sensitive compounds and if they are exposed to light for a sufficiently long time, then they change color from white/light yellow to purple/grey and on very long exposure they can even turn black.
More important though, it was discovered as well, that silver halide, which is exposed to light very briefly (and hence still is silver halide and still has a white/light yellow appearance) can be reduced by chemical reductors much more easily than silver halide, which never has seen any light. Other factors, such as the purity of the silver halide also appeared to have a strong influence on the ease with which they can be reduced to metallic silver.
No explanation could be given to this phenomenon, and even up to now, there still are aspects of this phenomenon, which are not fully understood. Although there was no explanation, the phenomenon was given a name, it was called latent image.
The name latent image was chosen because of the following: A sheet of paper, covered by a thin layer of a silver halide, when exposed to light, and then treated with a suitable chemical reductor shows strong differences in speed, at which the silver halide is reduced to metallic silver. The parts of the sheet, exposed to light, quickly turn black, while other parts, which have not been exposed to light turn black at a much lower rate. When such a sheet of paper is immersed in a solution of a chemical reductor, and it is taken out of that solution at the right time, then the pattern of exposure to light can be made visible as an image. The image apparently already was there, and is was made visible by means of selective chemical reduction. Hence the term 'latent image'.
Latent image -- modern explanation
Now, it is known what causes the latent image. When silver halide crystals are struck by photons of sufficient energy hv, then an electron is freed from the silver halide lattice and a so called 'electron hole' is created in the crystal lattice:
AgX + hv → [AgX]+ + e-
Here, [AgX]+ stands for the electron-deficient site in the crystal lattice. The electron can move away. In very pure silver halide crystals, the electron does not go far and it recombines with a 'hole' and the net effect is that nothing interesting happens. In the presence of impurities, however, the hole can be 'neutralized' by some other chemical agent, which easily gives up an electron:
[AgX]+ + impurity → AgX + reduced impurity species
If the hole is 'neutralized', then the electron e- is available for combining with another unit of AgX, located nearby:
AgX + e- → Ag + X–
So, in the presence of a suitable 'hole neutralizing agent', the capture of photons of sufficient energy results in a net reaction in which an electron is transferred from the 'neutralizing' agent to a silver atom. The latent image, hence is the presence of single metallic silver atoms in a 'sea' of silver halide, mixed with 'neutralizing' agent. The amount of free metallic silver is so low, that it is absolutely invisible, but as will be explained below, these free silver metal atoms play an important role in the development process, which transforms the latent image into a visible image.
Photographic emulsion, photographic sensitivity
Modern technology has pushed the effectiveness of the neutralizing agent to extreme heights. Modern photography uses a so-called emulsion, which is a mix of silver halides, gelatin and other (frequently kept secret) compounds. The gelatin acts as a base, in which the silver halide can be attached in a mechanically stable way to a sheet of paper, but it also acts as neutralizing agent for the electron holes. Other additives greatly enhance the effectiveness of neutralizing electron holes. The ideal photo paper has a neutralizing agent, which 'neutralizes' every hole [AgX]+ such that each photon of sufficient energy results in formation of an atom of silver metal.
Amplification, from latent image to visible image
With a latent image, present in an emulsion of silver halide/gelatin mix, there still is not a real visible image. The process of developing is used to make a visible image. This is described in the next web-page.