Silver halide crystals, composed of silver ions Ag+ and halide ions Cl-, Br-, or I- , are the light-sensitive components used in traditional black and white photographic film. The key photochemical properties of these crystals that contribute to the formation of an image on photographic film are as follows:1. Light sensitivity: Silver halide crystals are highly sensitive to light, particularly to the blue and ultraviolet regions of the electromagnetic spectrum. When the crystals are exposed to light, they undergo a photochemical reaction that forms a latent image on the film.2. Latent image formation: During the photochemical reaction, photons interact with the silver halide crystals, causing the reduction of silver ions Ag+ to metallic silver Ag0 atoms. These silver atoms accumulate at specific sites within the crystal, called sensitivity specks or latent image centers, which are typically imperfections or impurities in the crystal lattice. The accumulation of a few silver atoms forms a latent image, which is invisible to the naked eye.3. Developability: The latent image can be developed into a visible image by treating the film with a chemical developer. The developer reduces the remaining silver ions Ag+ in the exposed silver halide crystals to metallic silver Ag0 , amplifying the number of silver atoms at the latent image centers. This process forms a visible image composed of black metallic silver particles, which correspond to the areas of the film that were exposed to light.4. Selective reduction: The developer selectively reduces the exposed silver halide crystals containing the latent image while leaving the unexposed crystals unaffected. This selectivity is crucial for creating a high-contrast image with clear distinctions between light and dark areas.5. Fixing: After development, the unexposed silver halide crystals are removed from the film using a fixing agent, typically sodium thiosulfate or ammonium thiosulfate. The fixing agent dissolves the unexposed silver halide crystals, leaving behind only the metallic silver particles that form the visible image.6. Grain size and sensitivity: The size of the silver halide crystals affects the sensitivity and resolution of the photographic film. Larger crystals are more sensitive to light, allowing for faster exposure times and better low-light performance. However, larger crystals also produce a coarser grain, which can reduce the sharpness and resolution of the final image. Conversely, smaller crystals produce finer grain and higher resolution but require longer exposure times or more intense light sources.In summary, the photochemical properties of silver halide crystals, such as light sensitivity, latent image formation, developability, selective reduction, and grain size, play crucial roles in the formation of an image on traditional black and white photographic film. These properties enable the conversion of a light pattern into a visible image composed of metallic silver particles, allowing for the capture and preservation of moments in time.