CHAPTER IV 



THE ROLE OF INFRA-RED RADIATION 



Infra-red light comprises all the radiations of wave- 

 length greater than 8,000 A and these are invisible. Here we 

 are considering only the near infra-red, i.e., of wave-lengths 

 between 8,000 and 20,000 or 30,000 A. A httle more than half 

 of the solar radiation is in this region, most of the remainder 

 being in the visible range. 



Incandescent lamps, which are the most common artificial 

 light sources, radiate about nine-tenths of their power as 

 infra-red radiation. 



It is therefore important to know the particular action 

 of these radiations, both for plants grown in natural daylight 

 and for those — which are bound to increase in number — 

 cultivated wholly or partially in artificial light. 



Properties of the Infra-red 



In comparison with visible fight, infra-red is characterized 

 by a smaller quantity of energy in each of the quanta, or 

 grains of energy of which it consists. In fact, each quantum 

 carries an amount of energy /zv, proportional to the frequency 

 V, which is the number of wave-lengths contained in 

 300,000 km. Compared with a quantum of green light of 

 wave-length 5,000 A, a quantum of infra-red light contains 

 half the energy at double the wave-length, i.e., 10,000 A, one- 

 third at 15,000 A, and so on. The chemical activity of the infra- 

 red is therefore much less. 



In the language of chemistry, the absorption of a photon 

 of wave-length 5,000 A by a molecule corresponds to a "heat 

 of activation" of 56,900 calories. At the wave-length 15,000 A, 

 this heat would be reduced to one-third, i.e., 18,966 calories. 



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