The mechanism of photo-induced chemical reactions in silver halide crystals is based on the interaction between light and the silver halide crystals, which are typically silver bromide AgBr or silver chloride AgCl . These crystals are the key components in photographic materials, such as photographic film and paper.When light, specifically photons, strikes the surface of a silver halide crystal, it can interact with the crystal lattice and cause an electron to be excited from the valence band to the conduction band. This process creates an electron-hole pair, where the electron is now free to move within the crystal lattice, and the hole is a positive charge left behind in the valence band.The free electron can then be trapped by a sensitivity speck or a defect in the crystal lattice, which is usually a small cluster of silver ions Ag+ . This trapping process forms a silver atom Ag0 and leaves behind a positive hole in the halide ion Br- or Cl- . The silver atom is the beginning of the latent image, which is the invisible image formed on the photographic material upon exposure to light.As more photons interact with the silver halide crystals, more silver atoms are formed, and they begin to cluster together, creating a metallic silver nucleus. Once a critical size is reached, the silver nucleus becomes stable and can act as a catalyst for the subsequent development process.In the development process, a reducing agent developer is used to convert the remaining silver ions in the vicinity of the silver nucleus into metallic silver. This forms the visible image on the photographic material, with the metallic silver particles appearing as the dark areas of the image, while the unexposed silver halide crystals remain as the lighter areas.The photosensitivity of photographic materials is determined by the size and distribution of the silver halide crystals, the presence of sensitivity specks, and the composition of the emulsion. Smaller crystals and a higher concentration of sensitivity specks increase the photosensitivity, allowing the material to capture images with less light exposure. However, this can also result in a grainier image due to the increased number of silver particles formed during development.In summary, the mechanism of photo-induced chemical reactions in silver halide crystals involves the interaction of light with the crystal lattice, creating electron-hole pairs and forming latent images through the formation of metallic silver nuclei. This process is the basis for the photosensitivity of photographic materials, allowing them to capture and develop images upon exposure to light.