Response to chemical reaction

Basicaly, it's a preferential reduction of Silver Halide crystals to metallic Silver, by an organic reducing agent in an alkaline solution.
AgX crystals (AgBr with some AgI in film) that have been exposed to enough light form small deposits of metallic silver on their surface, and these 'latent image' specks allow the reducing agent in the developer to penterate the crystal lattice and so completely reduce it to Silver metal.
Those crystals that haven't been exposed have a protective outer shell of negatively charged ions around them, and this prevents them being reduced.
Some of the exact mechanism and chemistry of latent image formation and development is still not fully known. The gelatin emulsion plays a large part, by reinforcing the ionic protection of the AgX crystals, and contributing sulphur impurities which somehow enhance the speed of the film.
A lot of photographic chemistry is still more black art than pure science, I'm afraid.

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