134 



RADIATION BIOLOGY 



correspondence between photons absorbed and molecules reacted as 

 required by Einstein's law. This is not the result of failure of the law 

 but is due to complicating side reactions peculiar to each process. The 



Table 3-5. Number of (Quanta or Einsteins per Unit of 

 Radiant Energy for Various Wave Lengths 



measure of the effectiveness of the radiant energy is given by the quantum 

 efficiency, or quantum yield, <^ [denoted as y by Rabinowitch (1945)]: 



_ number of molecules reacting 



4> 



number of quanta absorbed 



In photosynthesis the reciprocal of the quantum efficiency !/</> is termed 

 the "quantum requirement" and is the number of quanta per reacting 

 molecule. 



QUANTITATIVE UNITS OF ENERGY 



The quantitative units of radiant energy fall into three categories: 



(1) energy, which is related to the total quantity of the radiant energy; 



(2) power, the time rate of flow of energy, or energy per unit of time; 

 and (3) intensity, the power per unit area, volume, and/or solid angle. 



There are two principal systems of metric units: the centimeter-gram- 



