3o6 THE CHLOROPHYLLS AND THE CAROTINOIDS 



Visible light, however, constitutes only a small part of the spectrum of 

 radiant energy (Fig. 79). Beyond the visible red lies the long zone of infra- 

 red or "heat waves" which range up to a wave length of about I00,000 /«/a. 

 Electric waves are still longer and range in length up to a kilometer or more. 

 The waves used for radio transmission are in this portion of the spectrum of 

 radiant energy. 



Just below the region of visible light in the radiant energy scale lies the 

 ultraviolet zone which ranges down to wave lengths as short as 10 Wju. Even 

 shorter are the X-rays, much used for their therapeutic effects in medicine. 

 Below them on the scale lie the gamma rays which are emitted by radium, 

 also used in medical therapy. Shortest of all are the cosmic rays, which are 

 less than o.OOOi mix in wave length. 



The wave lengths of the sun's radiation which reach the earth's surface 

 — much of the ultraviolet and infrared are absorbed by the blanket of atmos- 

 phere which envelopes the earth — range from about 300 ot/a in the ultraviolet 

 to about 2600 71111 in the infrared. Only a relatively small portion of this 

 range of wave lengths represents visible light. In their natural habitats plants 

 are also subjected to bombardment by the extremely long electric waves, and 

 the extremely short cosmic waves, but there is no experimental evidence that 

 either of these kinds of radiation has any effect upon plants. 



In the preceding discussion we have referred to radiant energy as a wave 

 phenomenon with an air of finality which is not justified by observed facts. 

 Many radiant energy phenomena, such as the behavior of light in optical 

 sj^stems, can only be satisfactorily explained at the present time in terms of 

 the postulate that light travels as waves. Other effects, however, appear 

 completely unintelligible in terms of this hypothesis. The most important 

 of these are photochemical reactions such as the effect of light upon sensitized 

 photographic paper and its role in the process of photosynthesis. At the 

 present time such phenomena can only be explained satisfactorily by the as- 

 sumption that light is particulate in nature. According to this concept a 

 beam of light is pictured as a shower of tiny particles. Each of these particles 

 is called a photon. When such photons impinge against a suitable substance 

 their energy may be transferred to the electrons which they strike, thus induc- 

 ing photochemical reactions. 



Each photon carries one quantum of energy. The energy value of quanta 

 varies inversely with the wave length. A quantum of ultraviolet radiation 

 with a wave length of 100 mp., for example, has four times the energy value 

 of a quantum of violet light with a wave length of 400 m/A, and eight times 

 that of a quantum of infrared radiation with a wave length of 800 inii. 



Radiant energy, therefore, apparently possesses a dual nature, and at 



