Ingredients: silver nitrate, sodium thiosulfate, tungsten light

Procedure: A partial recipe follows.

1. Prepare a solution of silver nitrate.

2. Paint a paper with silver nitrate solution.

3. Expose paper to bright light and observe reaction.

4. In a variation, prepare two papers, each covered with silver nitrate solution.

5. Using a mask, expose a limited region of each paper to bright light.

6. Rinse one paper with sodium thiosulfate solution and leave the other paper as is.

7. Submit the full surface of both papers to the bright light and observe the result.

Understanding: The chemical reaction of the silver salt with light leads to the photoreduction of silver ion to metallic silver

Ag⁺(aq) → Ag(s)

In the experiment using exposure through a mask, the full paper is coated with silver ion. Only the portion of the paper exposed to light is observed to darken, creating a contrast with the unexposed regions, and an image.

Our fully coated paper is similar to a piece of unexposed photographic film. A piece of photographic film might hold silver bromide salt in a gelatin. The film is exposed to a pattern of light. The light ionizes the bromide ion and the freed electron ultimately reduces a nearby silver ion. Some regions of the film are exposed to more intense light than others. The amount of silver reduced in a given region will be proportional to the intensity of light. That variation in intensity creates contrast on the exposed film, and an image.

Before the film can be exposed to more light, the unreacted silver ion must be washed from the film. That is accomplished during the fixing process when the film is treated with sodium thiosulfate solution known as photographic hypo. The hypo binds the unreacted silver ion, leading to the formation of the dithiosulfatoargentate complex ion

AgBr(s) + 2 S₂O₃^2-(aq) → Br^-(aq) + Ag(S₂O₃)₂^3- (aq)

Note that this process creates a negative of the image. It is the brightest regions that lead to the darkest portions of the image. In the printing process, the image is corrected to create a positive with the contrast and brightness of the original image.

Before the modern photographic process, there was the daguerreotype. To make a daguerreotype, a copper plate covered with metallic silver is treated with iodine vapor, creating light-sensitive silver iodide. Exposure to light reduces the silver ions. The plates are developed using mercury vapor; the mercury combines with silver to create a milky-white amalgam. The milky-white amalgam of silver and mercury creates a visible contrast with the dark copper plate. The images are "fixed" by rinsing away the silver ion using a solution of a common salt, followed by hot distilled water. In the process of creating a daguerreotype, we produce no negative. The positive image is created directly on exposure of the prepared plate to light.