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How does black-and-white film record an image using silver salts?

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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).

Black-and-white film records an image through a chemical process involving silver halide crystals—typically silver bromide—suspended in a gelatin emulsion.12

When light enters a camera and strikes these crystals, it triggers a reaction that creates an invisible latent image. Specifically, incident photons release electrons from the silver halide crystals, which migrate to surface defects known as sensitivity specks. These negatively charged specks attract interstitial silver ions, which accumulate and form stable clusters of metallic silver atoms.13

This latent image remains invisible until chemical development. During development, the silver clusters act as catalysts, allowing a selective reducing agent (the developer) to convert the entire exposed silver halide crystal into metallic silver. This creates the dark specks that form the visible image. Finally, a fixing bath removes the unexposed silver halide, leaving behind the permanent silver image.314