106 LIGHT, VEGETATION AND CHLOROPHYLL 



more rapid rate — an analogy with a chemical reaction which 

 is performed with greater speed. 



We now pass on to the photochemical action of light. 

 The luminous energy is concentrated in photons or hght 

 quanta ; each of them carries a quantity proportional to the 

 frequency of the luminous vibration. For green mono- 

 chromatic light, for example, of which the wave-length is 

 5,500 A (consequently the frequency v=0'545xlOis), the 

 energy E of a photon is E=/zv (where /7=6-554 x 10— 2^), so that 

 E=(6-554 X 10-27) (0-545 x 10i5)=3-58 x lO-i^ ergs. 



This energy is about ninety times greater than the average 

 thermal energy of a molecule at ordinary temperature. If 

 such a molecule receives and absorbs the energy of a photon, 

 it will therefore be brought to a high energy level and strongly 

 activated. It will become capable of chemical reactions which 

 would be impossible by thermal activation alone. 



The billiard balls individually receive violent impulses 

 which in all probability will throw them off the table. 



Because photochemical activation operates by supplying 

 energy, it can make endothermic reactions possible — reactions 

 which absorb energy instead of liberating it, as we have 

 previously supposed. 



The impulse received by a ball throws it up to a higher 

 level, where it will remain if it finds, for instance, a shelf 

 higher than the table; molecules confronting one another 

 combine, giving a compound of which the chemical energy 

 is greater than the energy of the constituents. 



This is exactly what happens in photosynthesis by green 

 plants; while in the majority of photochemical reactions 

 Hght is only a stimulus, in assimilation through chlorophyll 

 the luminous energy is used not only to "activate" the 

 molecules taking part, but also to fix this energy, in the form 

 of chemical energy, in the substances created. 



It would seem, according to the process outUned above, 

 that a molecule participating in the reaction corresponds to 

 each photon absorbed. 



But complications often arise from simple phenomena. 



