48 



LIGHT, VEGETATION AND CHLOROPHYLL 



What becomes of the light which strikes a material surface, 

 such as a leaf? It is divided into three parts : the first is reflected, 

 usually in a diffused way; the second is transmitted after 

 having traversed the thickness of the leaf; the rest ceases to 

 exist in the form of light, its energy being used and changed 

 into another form (Fig. 1, 10). Only this third part entering into 

 the physical and chemical processes of the vegetable tissues 

 is of interest to us, but we have no means of measuring it 



Incident light 



Reflected light 



Transmitted 



Fig. 1, 10. Interaction between light and leaf. Part of the 

 light is transmitted or reflected; the difference between 

 that part and the incident light gives the part absorbed by 



the leaf 



except by finding the difference between the incident Ught on 

 the one hand and the sum of the reflected and transmitted 

 parts on the other. 



That is why it is important to measure exactly the light 

 which is reflected and transmitted by the leaves, whether their 

 surface is left naturally uncovered or is coated with chemicals 

 or powders modifying, intentionally or not, the proportion 

 of Ught absorbed. 



To calculate the fraction of light reflected we need to 

 know: (1) the composition of the incident light; (2) the 

 reflection factor for each of its monochromatic radiations. 



