18 LIGHT, VEGETATION AND CHLOROPHYLL 



each of these waves is measured with deUcate instruments, 

 thermocouples or bolometers, whose temperature rises as 

 they absorb the energy and transform it into heat; it is this 

 rise in temperature which is measured. 



The monochromator can isolate, in the exposed spectrum 

 of light, not only visible waves but other waves, on both sides 

 of the visible, which also transport a measurable quantity of 

 energy. These waves, called infra-red if they are less refracted 

 by the prism than the visible, and ultra-violet if they are more 

 refracted, exist in solar radiation, the normal source of light 

 for vegetation, and are nearly always present in the light 

 given by ordinary lamps. 



The fact that they have no effect on our retina and are 

 invisible to us does not at all imply that plants are indifferent 

 to them. A description of the light received by a plant must 

 therefore necessarily include not only the radiations which 

 excite the human retina but also the electromagnetic waves of 

 the same nature, ultra-violet and infra-red. 



The quality of a monochromatic radiation is wholly 

 determined by its wave-length, which is expressed in microns 

 (1 /x=0-000001 m.) or in Angstroms (1 A=0-0001 micron). 

 For example, the wave-length of yellow sodium hght, which is 

 nearly monochromatic, is 5,893 A, or 0-5893 /x. 



Maxwell's experiments showed that light, visible or 

 invisible, is an electromagnetic radiation. It is a double electric 

 and magnetic field. The small electrified particles in matter, 

 such as electrons, come under the influence of these waves 

 and in turn react on them, converting all or part of their 

 energy into another form and retarding their propagation, 

 thus giving rise to the two classic phenomena of absorption 

 and refraction. 



Photons or Quanta 



The properties of electromagnetic radiation are often 

 perfectly represented by means of the v/aves of which we 

 have just spoken. Other properties indicate that this radiation 

 consists of a shower of particles, or grains of energy, called 



