The component that gets the developing process going is the reagent (as in re-agent), a mix of opacifiers, alkali, white pigment and other elements. The reagent sits in a layer just above the light-sensitive layers and just below the image layer. Before you take the picture, the reagent material is all collected in a blob at the border of the plastic sheet, away from the light-sensitive material. This keeps the film from developing before it has been exposed.
After you snap the picture, the film sheet passes out of the camera, through a pair of rollers. The rollers spread the reagent material out into the middle of the film sheet, just like a rolling pin spreading out dough. When the reagent is spread in between the image layer and the light-sensitive layers, it reacts with the other chemical layers in the film. The opacifier material stops light from filtering onto the layers below, so the film isn't fully exposed before it is developed.
The reagent chemicals move downward through the layers, changing the exposed particles in each layer into metallic silver. The chemicals then dissolve the developer dye so it begins to diffuse up toward the image layer. The metallic silver areas at each layer -- the grains that were exposed to light -- grab the dyes so they stop moving up. Only the dyes from the unexposed layers will move up to the image layer. For example, if the green layer was exposed, no magenta dye will make it to the image layer, but cyan and yellow will. These colors combine to create a translucent green film on the image surface. Light reflecting off the white pigment in the reagent shines through these color layers, the same way light from a bulb shines through a slide.
At the same time these reagent chemicals are working down through the light sensitive layers, other reagent chemicals are working through the upper film layers. The acid layer in the film reacts with the alkali and opacifiers in the reagent, making the opacifiers become clear. This lets you see the image below. The timing layer slows the reagent down on its path to the acid layer, to give the film time to develop before it is exposed to light.
When you watch the image in a photo film come into view, you're actually seeing this final chemical reaction. The image is already developed underneath -- you're just watching the acid layer clear up the opacifiers in the reagent so the image becomes visible.
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