CHAPTER XIX 



THE CHLOROPHYLLS AND THE CAROTINOIDS 



Radiant Energy. — An elementary knowledge of the physical properties 

 of light and other forms of radiant energy is essential for a proper under- 

 standing of photosynthesis, the synthesis and properties of chlorophyll, and 

 many other plant processes to be discussed in the subsequent chapters of this 

 book. Radiant energA^ as judged from some of its properties, appears to be 

 propagated across space as undulatory waves. Ordinary sunlight or "white 

 light" from any artificial source seems homogeneous to the human eye but after 

 it has passed through a prism appears as a spectrum of colors. This dispersion 



390 



100000 m/; 



390 4 30 470 500 560 600 650 



Fig. 79. The spectrum of radiant energy. 



750 m/u 



of light by a prism was first demonstrated by Newton in 1667, but man had 

 already long been familiar with the similar phenomenon which occurs in 

 rainbows. The order of the more prominent colors in a spectrum of sun- 

 light is red, orange, yellow, green, blue-green, blue, and violet. Each of these 

 colors corresponds to a different range of ivave lengths of light (Fig. 79). 

 The wave length is the distance between two successive crests of a wave. The 

 wave lengths which induce photochemical reactions in the retina of the human 

 eye that result in the sensation of light, range from about 390 mfi to about 



1 A millimicron {nifx) is one-thousandth of a micron. 



305 



