LIGHT AND VEGETATION 19 



photons or quanta. The energy of each photon is proportional 

 to the frequency of the wave, the coefficient of proportionaUty 

 being the well-known universal Planck's constant, h. Between 



the wave-length A (or the frequency v = -) and the energy E 



A 



of a photon there exists the very simple relationship 



E = /zj, = h- 



A 



where c is the velocity of light in a vacuum. 



We shall then ask, "Does hght consist of waves or 

 particles? Must we imagine continuous spherical waves 

 expanding round the source of Hght, or a shower of discrete 

 particles, projected in straight Hues in all directions?" 



Radio-waves 



Infra-red 



Visible rays 



./ Ultra-violet 



- / X-rays 



. > ■ I ■ I ■ ' ■ ' ■ ' ] 



Fig. I, 1. Scale of electromagnetic radiations. X: wave- 

 length in microns, v. frequency in cycles per second 



The physicist will reply that the two aspects are equally 

 indispensable; they are prescribed by experimental obser- 

 vation for light as well as for all electromagnetic radiations. 

 Sometimes we shall employ terms applicable to waves, some- 

 times those apphcable to photons, passing freely from one 

 mode of expression to the other, since they represent two 

 aspects of the same phenomenon, which is beyond our range 

 of imagination and to which our words and familiar concepts 

 are ill adapted. 



These are the properties, not only of light, but of a con- 

 tinuous series of radiations which extend over an extra- 

 ordinarily large range of wave-lengths and which are also 

 electromagnetic. Fig. I, 1 shows them in diagrammatic form. 



The visible radiations occupy an extremely small portion 



