What do visible light photons cause during film development?

During the film development process, visible light photons cause the film to darken. This occurs because photographic film contains a light-sensitive emulsion that reacts when exposed to light. The emulsion contains silver halide crystals, which are sensitive to light. When these crystals are exposed to visible light photons, a chemical reaction happens that forms metallic silver in the areas that were exposed.

As a result, the areas that received more light will appear darker once developed, while those that received less light will remain lighter or clear. This principle underlies the entire process of traditional film photography, where exposure to light contributes directly to the final image's tonality and contrast. Thus, understanding the role of visible light in film development is essential for anyone working with photographic processes.