An emulsion holding grains of photosensitive chemical compounds called silver halides is spread over a film or other material. Light coming through the camera lens from an object being photographed strikes certain areas of the film, rendering the silver halide grains in those areas unstable. This creates an invisible, or latent, image of the object on the film. The areas of the latent image that receive the most light contain the largest number of unstable grains. Upon development they become the darkest areas of the visible image. Conversely, areas that receive little light form the bright parts of the visible image.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.