When German professor Johann Heinrich Schulze began to experiment with silver nitrate’s reaction to the light, it can easily be said that his late 1700’s studies paved the path for something absolutely revolutionary. These silver nitrate salts were known to darken and turn black when exposed to light. Perhaps the most incredible thing to emerge from Schulze’s experiments was a single idea.
This idea belonged to Nicéphore Niépce, a French inventor. Niépce’s hoped to create visible images of his surroundings using this silver nitrate and the light around him. This sudden scientific breakthrough lead to more and more development, eventually leading to the introduction of photography. Through the studies of silver nitrate done by several innovators, such as Louis-Jacques-Mandé Daguerre, Thomas Wedgwood and Humphry Davy, the ideas and possibilities began to expand rapidly.
With these ideas came the first photograph, credited to Niépce in 1820, thus beginning the amazing introduction of photography to the world. Without photography, the world today would be incredibly different. It has been used to document history and capture fleeting moments in ways that murals and portraits cannot. However, without the ideas of many great chemists, this important accomplishment would not exist. The chemical reactions that lie within photography intertwine and come together to form something amazing.
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To fully understand the chemical reaction of photography, it is important to first understand the structure of the film that these images are etched into. The three major parts of the film’s structure are the silver halide crystals, gelatin, and plastic backing. The silver halide crystals are incredibly important. These crystals react by darkening significantly when exposed to sunlight. Without this reaction, there would be no image created from the absorption of photons.
The presence of the silver halide crystals allows the image exposed to be printed onto the film through decomposition. However, silver halide crystals are not exclusively used in photography. For example, two-for-one eyeglasses contain the crystals. When in the sunlight, the glasses darken, becoming clear once more once returning in-doors. These crystals are suspended in a gelatin, and they remain held in the medium through the process.
Gelatin has been used as the medium for photographs for over one hundred years due to the cost of other mediums that could be alternatively used. What holds it all together is the plastic backing of the film. It acts as a backbone for the gelatin and silver halide crystals, supporting what causes the reaction and, ultimately, the final result of a picture.
Although the silver halide’s darkening reaction has been explained, the question still remains; Why does the silver halide blacken when exposed to light in the first place? To find an answer, the structure of the crystals themselves must be observed. These silver halide crystals, the halide usually being chlorine, bromine, or iodine – bromine being the most common, are structured in a crystalline fashion, meaning that the structure is neat and ordered in a three-dimensional fashion. This is what the structure looks like:
Once the grain of silver halide comes in contact with light photons, an electron from a halide ion is ejected. This dislodged electron will find its way into a the conducting band of the crystal. Since there are different energy levels and charges between the two bands, the ejected electron will latch onto and combine with the silver ion. Due to the positive charge of the silver ion and the negative charge of the electron combining, the atom becomes neutral.
This ion migration forms atomic silver, a clump of atoms that combine to form a tiny speck that then becomes a latent image. The latent image is formed as all of the individual tiny specks form by the million. When the picture is taken and the light photons stream in through the lense, the immediate amount of darkness or light in certain places result in the final image’s appearance. Areas with more light create more atomic silver, which results in more darkness. The process looks like this:
Although the millions of less-than-microscopic black dots come together, the latent image still is not visible to the naked eye. The dark atomic silver will create the dark areas that can only be seen after development of the picture.
Once the chemical reaction of transferring the image onto a film is completed, there is only one step left; the development of the film. This is necessary, because the developer “brings out” the crystals, making their dark areas of atomic silver visible to the eye. In the image, the lighter areas appear dark because of the light photons, although this is much different than what was directed in front of the camera’s lense in the first place.
This is because the image, due to the atomic silver’s abundance where the light is most intense, is shown through a negative. Areas that may have been originally dark appear brighter, and light areas appear darker. The developer, however, will begin to bring out the image to its original appearance. As development takes place, it does so in a “dark room.”
The lights in this room are red, which is the only hue of light that does not cause the silver halide crystals to react, therefore preserving the image. This developer must be high in acidity, common chemicals used being hydroquinone, dimezone, and phenidone, so strong bases such as sodium hydroxide are added. As the image develops, it is finally able to be seen in clear black and white. If the photographer wants the result to be colored, couplers can be added with the silver halide and suspended in the gelatin. These color couplers provide chemicals that provide dye, giving the image its original color.
These dyes are also brought out during development, alongside the rest of the image. As the developer continues to bring out this image, it will continue to do so, bringing out all of the silver halide. This is known as overexposure. In order to avoid this, the film, once developed to its desired result, must be placed in a stop bath. The stop bath is usually composed of acetic acids, which neutralizes the developer, stopping it from bringing out any more of the silver halides. Although the stop bath could even be composed of water, the acetic acid is much more reliable and thorough, assuring that the image has been completely preserved.
Through the crystalline structure of silver halide, the reactions to light photons, and the development of the ionic silver, the process of photography is a series of intricate reactions that interact with one another, ultimately resulting in a perfect print of one’s surroundings. Since the development of photography, the expansion and importance of photography has only continued to grow and swell. Although it began as silver nitrate studies, photography has grown into something incredible.
Chemistry has enabled photography to expand from developed film, eventually leading us to where we are today. Modern day photography has become so advanced that a single picture can be taken in mere seconds, the technology of clear photography at the fingertips of millions. Without chemistry, this simply could not exist. Looking back at Niépce’s development of the first photograph in 1820, it is safe to say that the current outcome of photography is far from anticipated or expected. Chemistry and the development can lead to truly remarkable accomplishments, and photography is no exception.
