Chemical effects of light

To complement the research efforts targeted at elucidating and controlling the physical characteristics of light, active research was also conducted on the chemical effects of light, dating back to the early decades of the eighteenth century. In fact, the roots of the science of photochemistry date back to this era, when in 1727 Johann Heinrich Schulze (1687-1744) (Fig. 3.15) discovered that if a bottle of silver nitrate crystals was placed in the sun and part of the bottle 

In 1777, Karl Wilhelm Scheele (1742-1786) (Fig. 3.16) published the first comprehensive book on the chemical effects of light. Considered the father of 

The science of photometry— the measurement of the effect of light intensity whether transmitted or absorbed on various materials— provided another impetus for research on chemical effects of light during this period. These studies had broad focus, spanning research on astronomy and chemical effects of light. Very basic photometers were made with paper saturated with silver nitrate, silver chloride, or other light-sensitive solutions.  

University of Chicago Press, Chicago (1987). 

In addition, spectral light was used to study gases and flames colored by the addition of various metal salts and transparent liquids, as well as to monitor the effects of photochemical changes. Based on his work on spectral analysis, Scheele established that there is a difference between actions of light and heat. This line of research was subsequently to lead to the discovery of the infrared region of the spectrum in 1800 by Friedrich Wilhelm Herschel (discussed above), and the ultraviolet region in 1801 by J.W. Ritter (discussed above). It also led William Hyde Wollaston to discover the ultraviolet region of the spectrum in 1802 he referred to this discovery as chemical rays.  

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Earlier we defined the terms primary photochemical process and secondary reaction. Another term of great significance when dealing with the chemical effects of light is primary photochemical yield. This is the number of absorbing molecules which either dissociate or react per photon absorbed. The primary quantum yield may not exceed unity and it may be zero, if, for example, all absorbing molecules fluoresce without reacting (14). 

Photochemistry The branch of chemistry concerned with the chemical effects of light (far UV to IR). 

Bergman found that silver and mercurous oxalates blacken in sunlight. Scheele found that metallic silver is formed in silver chloride blackened by light, and that it grows black sooner in the violet than in any of the other rays , the first distinction of the different chemical effects of light of different colours. He seems to have known of Schulze s work. ... 

As mentioned, the observation of chemical effects of light is as old as mankind itself. Accounts of the earlier history of photochemistry have been presented by several authors. Long erudite lists of reference are of little usefulness, however. Here, it has been chosen to present a brief historic profile based on a document itself of historic value, the introduction written by a great authority, professor Ivan Plotnikov. This well-known Russian-bom scientist published in 1910 a book on photochemistry which was followed in 1936 by a second, much extended, edition of over 900 pages. The view he had on this science, or at least on some aspects of it, in particular the unconditioned refusal of the Stark-Einstein equivalence law, appeared obsolete at the time of the second edition, as it is discussed in Sect. 2.3. However, Plotnikov can certainly not be accused of insufficient knowledge of the matter or of insufficient exploration of the literature and his book is a rich mine of data and thoughts [7]. 

This paper prompted a note by Stark [35], who remarked that in a paper of his in 1908 [36], he had stated that the direct chemical effect of light consists in the separation of valence electrons from their bonds and had arrived to the following statement the amount of substance primarily reacting upon light absorption is proportional to the intensity of the active light, since both the reacting amount of... 

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