When and image is exposed onto a piece of  light sensitive material like film, the silver salts are affected by light and an invisible image is retained on the film.

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This invisible image retained on the film before development is called the 'Latent Image' and remains on the emulsion, in some cases for years. 
 

As the darkening action of the silver salts exposed to light takes an extended time, a developer is used as an accelerator to convert the silver salts to metallic silver and materialize the image. Silver halides are compound salts, i.e. silver bromide, iodide and chloride. The remaining silver salts underdeveloped are removed with an acid fixer to reveal a transparent base, producing the tonal variations that make a negative representation of the scene that was photographed.

(For the Disturbance in the field work, I use a projector to not only project a shadow image onto the photographic paper, but to develop the image through the action of light.To produce this image only through the actions of light took 2 weeks.