20 LIGHT, VEGETATION AND CHLOROPHYLL 



of the electromagnetic spectrum, but it is of the highest 

 importance to us since it is the only part to which our eye is 

 sensitive; in addition it contains the active rays for photo- 

 synthesis and rather more than half of the solar emission is 

 situated in this band of wave-lenths. 



It is, however, the larger band, lying approximately 

 between 1,850 and 40,000 A, that we shall call "light", thus 

 extending the meaning of this word which was formerly 

 limited to the visible rays. It has become customary to speak 

 of ultra-violet or infra-red light although these rays are 



^ OJ85 0-4 0-75 4 pL 



Ultra-violet Visible Infra-red 



V 1621 750 450 75 



Fig. I, 2. Scale of visible and invisible rays the effects of 

 which are discussed in this book 

 a: wave-length in microns 

 v: frequency in 10^2 cycles per second 



invisible. Fig. I, 2 gives the boundaries of the spectrurn of light 

 with which we are now concerned. 



Absorption of Light Rays 



The essential property of light, to which we shall often 

 return, is its transport of energy. This energy can be measured 

 in joules or in kilogram-metres, in kilowatt-hours or in 

 calories. 



Light from the sun has traversed interplanetary space, the 

 earth's atmosphere and, sometimes, sheets of glass, before it 

 reaches us ; it has thus been filtered and deprived of certain 

 parts which have been absorbed. One of the properties to be 

 studied is therefore the transparency of various media to 

 diff'erent radiations. 



Although fight is propagated in a vacuum without loss of 

 energy it is not so propagated in matter — nearly all bodies 

 are opaque to the radiations that we have agreed to call 



