In the case of the silver iodide molecule you give it a good wind up of light energy. Its sort of like setting a bear trap. Your are putting your energy into pulling the trap open. Now its set to snap shut. The same thing is happening in the silver iodide molecule.
Light gives it energy to be ready to snap onto another atom.
When you put it in a bath of photographic developer, it grabs hold of the oxygen in the bath, and tarnishes. That's why black and white negatives are black. And so are holograms before they are bleached. So, you can think of photography or by extension holography as the art of selectively tarnishing silver in jello where light has energized it. Now in the case of a hologram, the patterns of light wave impressions are what is being photographed in the layer of emulsion.