PHOTOCHEMISTRY 



261 



as are present in colloid systems. In such cases a considerable quantity 

 of light may be scattered so that the amount absorbed by the system 

 and the amount transmitted will not necessarily be equal to the total 

 incident light. 



The photochemical law of greatest theoretical importance is that of 

 Einstein, according to which one molecule is activated for each quantum 

 absorbed. It does not necessarily follow that one molecule will react for 

 each quantum absorbed because there may be many complicating second- 

 Absorption of Light by Acetone Vapor 



600 



(A 



E 

 E 



.500 



400 



s. 



in 

 in 



a. 



300 



200 



00 



'0.6 



2.0 



0.8 1.0 1.2 1.4 1.6 1.8 



Logarithm of Per Cent Transmission. (log-y-) 



Fig. 5. — Graph illustrating Beer's law. The absorption of light by 10 cm. of acetone vapor. 



ary reactions. In fact, these secondary effects so frequently mask the 

 primary process that the Einstein law was not accepted at first. The 

 ratio $ of molecules reacting to quanta absorbed is known as the quantum 

 yield and the experimental determination of this quantum yield under 

 different conditions constitutes one of the important approaches to the 

 theoretical study of a photochemical reaction. The number of cases in 

 which the quantum yield is unity is very small and the more refined 

 experimental measurements of the last few years have not increased the 

 number. The quantum yield varies all the way from a few hundredths 

 or less to a million or more. These enormous variations serve simply to 

 emphasize the fact that the primary process is almost always followed by 

 other complicating phenomena. Most photochemists accept the view 

 that the Einstein relationship applies to the primary process. 



QUANTUM THEORY 



The quantum theory has been responsible for extraordinary develop- 

 ments in the fields of physics and physical chemistry. It was first pro- 



