Photochemistry and Photography

Rain or snow might be made to form by seeding with crushed ice. It is hard, however, to crush ice to a powder, and hence no large number of seeds can be distributed. It was discovered by the American scientist Irving Langmuir that minute crystals of silver iodide, formed by condensation of silver iodide vapor, can serve as seeds for ice crystals. This discovery is the basis of the silver iodide method of making rain or snow by seeding supersaturated parts of the atmosphere. 

Many chemical reactions are caused to proceed by the effect of light. For example, a dyed cloth may fade when exposed to sunlight because of the destruction of molecules of the dye under the influence of the sunlight. Reactions of this sort are called photochemical reactions. A very important photochemical reaction is the conversion of carbon dioxide and water into carbohydrate and oxygen in the leaves of plants, where the green substance chlorophyll serves as a catalyst. 

One law of photochemistry, discovered by Grotthus in 1818, is that only light that is absorbed is photochemically effective. Hence a colored substance must be present in a system that shows photochemical reactivity with visible light. In the process of natural photosynthesis this substance is chlorophyll. 

The second law of photochemistry, formulated in 1912 by Einstein, is that one molecule of reacting substance may be activated and caused to react by the absorption of one photon. In some systems, such as material containing rather stable dyes, many photons are absorbed by the molecules for each molecule that is decomposed the fading of the dye by 

These chlorine atoms initiate a chain of reactions, as described in Section 10-6  

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BECQUEREL EFFECT, A photographic effect discovered by E, Becquerel (1895). Experimenting with the daguerreotype process. Becquerel found that a plate will produce a direct (positive) image if exposed first to diffuse daylight. See also Photochemistry and Photolysis and Photography and Imagery. 

The photochemistry and photophysical studies of cyanine dyes are an important research field owing to the economical importance of these substances in color and black and white photography, in dye lasers, and as potential sensitizers for photodynamic therapy. 

Some of the major industrial applications of photochemistry are found in the various imaging processes which include photography, photopolymerization/ photodepolymerization, photochromism and electrophotography— the process used in photocopying machines. 

The most important and widespread applications of photochemistry are, of course, photoimaging techniques, such as photography and xerography (Special Topic 6.32).1313 Some other large-scale applications, such as environmental remediation of anthropogenic pollutants by photocatalytic processes (Special Topic 6.28) and potential applications such as photoelectrochemical cells (Special Topic 6.31) and artificial photosynthesis (Special Topic 6.26), are also discussed in the following text. 

As we now understand this phenomenon, absorption of a photon leads to an electronically excited state, characterized by high energy and a strongly perturbed electronic structure. The result often is an in-depth—but selective—chemical transformation. As everyone can easily appreciate, photochemistry has a key role in nature (e.g., chlorophyll photosynthesis, vision) and in a variety of applications. Photography and other methods of reproducing an image, the fast hardening of varnishes and of dental cement, and many other applications are based on photochemical reactions. 

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