Chemistry is behind one of the most important parts of our lives. Photographs are made available to us through a chemical reaction and change in composition. An article found at http://www.chemistry.org/portal/a/c/s/1/feature_ent.html?id=99c10f92ce1711d5f2944fd8fe800100 explains this process. The history of photography begins with Joseph-Nicephore Niepce who is known to have taken the first photograph in 1827. He started by using an asphalt solution and coating a pewter plate with it, which he then slid into a 1ft wooden cube. When he removed the plate eight hours later, he removed the excess asphalt solution and found a clear image of the house facing his, a barn, and a pear tree. He later found a way to put the image on a metal surface. Niepce partnered with Louis Daguerre who after Niepce's death was able to use silver halide in the process.
Silver halide is made using silver nitrate and halide salts and the result is a crystal of different shapes and sizes, all of which are held together by 20 layers of gelatin, called emulsion. This emulsion is suspended on a plastic strip and has a scratch-resistant coating, which makes silver gelatin. When light strikes the silver gelatin, say when the camera shutter opens, the silver halide grains suspended in the emulsion turn into pure silver atoms. The silver atoms are unstable and will form a lattice with each other thus creating an image. This image is invisible so one must convert all of the silver ions into silver metal via a developing agent. The transparent silver halide crystals are parts of the frame that did not capture any light and are removed. This creates a negative and the darkest area has the highest density of silver atoms because it received the most light and the opposite for the lighter areas.
A projector shines light through the negative onto chemically-treated paper. Silver ions block the light from reaching the paper, thus the darker area is white in the positive and opposite for the lighter area. This creates an image and is stored away to remind us of the good times of the past and present.
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I developed a case study on photography for a non-majors class I taught at Georgia Perimeter College. The class was required to develop a procedure for preparing silver salt prints. It was really cool. I have not used the case study at Emory because it is difficult to properly implement case studies with large enrollment classes (gen chem). I will post the exercise to our LL conference.
This is really interesting. At my high school I took a few photography classes and we developed all of our film by hand. I never knew what all of that dipping the paper in different smelly solutions was for (except that it made the picture show up). I wonder how color film works.
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