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Why do bright areas of a photographed subject become dark areas on a film negative?

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Diagram showing the negative-positive photographic process: a scene is captured on film in a camera, developed into a negative, projected onto paper with an enlarger, and developed again to produce a positive print. — technology of photography Figure 1: Sequence of negative–positive process, from the photographing of the original scene to enlarged print (see text).

In the technology of photography, bright areas of a subject become dark on a film negative because of the way light interacts with the film's emulsion during exposure and development.12

When a camera lens projects an image onto the film, light reflected from the subject strikes the silver halide crystals in the emulsion. Areas of the subject that are bright reflect more light, causing a greater number of these crystals to become "developable" by forming a latent image.32

During the development process, a chemical developer reduces these exposed silver halide crystals into metallic silver. Because bright areas of the subject caused more crystals to be exposed, they result in a denser deposit of metallic silver on the film. Conversely, dark areas of the subject reflect little light, leaving fewer crystals to be converted into silver, which results in areas of low density on the negative. This tonal reversal—where bright subject details record as dark, dense areas—is the defining characteristic of a negative.21