The chemistry behind developing film photos

An introduction:

When we are taking a photograph with our camera, we are recording the visible light reflected from the objects in the camera's field of view. In order to do that, the reflected light causes a chemical change to the photographic film inside the camera. Light's energy is distributed as photons. It is the energy in each photon of light that causes a chemical change to the photographic detectors that are coated on the film. The process whereby electromagnetic energy causes chemical changes to matter is known as photochemistry. 

The film:

The film is coated with 20 or more layers of gelatin which are less than one thousandth of an inch thick. Some of the layers coated on the transparent film do not form images. They are there to filter light, or to control the chemical reactions in the processing steps. The imaging layers contain sub-micron sized grains of silver-halide crystals that act as the photon detectors. They undergo a photochemical reaction when they are exposed to various forms of electromagnetic radiation -- light. These silver-halide grains have been chemically modified on their surface to increase their light sensitivity.  Organic molecules known as spectral sensitizers were added to the surface of the grains to make them more sensitive to blue, green and red light, since the unmodified grains are only sensitive to the blue portion of the spectrum, and they are not very useful in camera film.  

Developing colour films: An explanation

Light gets reflected off surfaces and through the lens of a camera, past an open shutter, and falls onto a piece of plastic coated with layers of light-sensitive emulsion. Silver halide crystals in the emulsion react by forming clusters of silver ions, creating a latent image. When the film is submerged in developer, it transforms the silver ions into pure silver, leaving behind the silver halide crystals that were not struck by light. Those excess silver halide crystals are washed away with a second chemical, leaving metallic silver grains that are denser where the light was more intense during the exposure, producing a visible negative image. The negative images are then turned into positive images.

Key words explained:

1) photographic detectors - sensors of light or other electromagnetic energy.

2) gelatin - a translucent, colorless, brittle, nearly tasteless solid substance, derived from the collagen inside animals' skin and bones.

3) negative images - A positive image is a normal image. A negative image is an inversion of a positive image, in which light areas appear dark and vice versa. A negative color image is also color reversed, with red areas appearing cyan, green appearing magenta and blue appearing yellow.