CHAPTEK XIX 

 THE ACTION OF LIGHT 



SINCE the whole existence of living organisms on the earth depends on the receipt 

 of radiant energy from the sun, it is unnecessary to point out the importance of 

 the study of the way in which this energy is made use of. If it were merely 

 converted into that form of energy in material bodies which we know as heat, it 

 would be a very wasteful process, as our study of energetics has taught us. Much 

 of the free energy would be thereby lost in the process of conversion to other 

 forms. A considerable part of the sun's energy is, of course, used up in warming 

 objects on the earth, but the study of the chemical reactions brought about by the 

 action of light is of greater importance, although of some difficulty. 



The amount of energy actually received from the sun may be somewhat realised by the 

 following data : Suppose that the atmosphere were absent and the sun in the zenith, then 

 each square centimetre receives per minute 1 '955 small calories, expressed in heat units. The 

 presence of the atmosphere, which absorbs a part of the radiations, reduces the value to l~2 

 small calories, at the latitude of Cambridge. In other words, the energy received by 1 sq. m. 

 (10,000 sq. cm.) in one minute would suffice to raise the temperature of a kilogram of water 

 by 12. 



The reader will not need to be reminded that the most important of all 

 photo-chemical reactions is that by means of which the chlorophyll system of the 

 green plant stores up light energy and, in the process, restores to the atmosphere 

 the oxygen which has been used up in oxidation by living beings. The stores of 

 energy in coal and petroleum also owe their origin to chlorophyll in past ages, 

 indirectly in the latter case, if we accept its animal origin. 



In order that we may be in possession of the means of understanding, as far as is possible 

 at present, the mechanism of the process concerned, we must first enter somewhat fully into 

 the general theory of photo-chemical reactions. In addition to the chlorophyll system, there 

 are other actions of light which are of physiological importance. Such are the retinal process 

 and the action of ultra-violet light. In practical use, the various photographic methods may 

 be mentioned, as well as those of wireless telegraphy, which makes use of waves like those 

 of light, but of very much longer wave length. 



ABSORPTION OF LIGHT 



All substances absorb radiant energy to some extent ; glass itself absorbs rays 

 of longer wave length than those we know as light, and also those of 

 shorter wave length. The colourless substance, anthracene, absorbs ultra-violet 

 rays, as is shown in the photograph of Fig. 176, and many other instances might 

 be given. In other words, a part of the energy of a beam of light which tra verses 

 any substance is removed and held back in the substance. Something must, 

 therefore, happen to the substance ; it may be merely warmed, or other forms of 

 energy may make their appearance in it, chemical or electrical change, and so on. 



Grotthus's Law. It seems fairly obvious to us at the present time that no 

 effect can be produced by light unless it is absorbed. We shall see presently also 

 that some light energy must be used up to start any photo-chemical change, even 

 when the reaction afterwards proceeds with evolution of energy. The law that 

 light must be absorbed in order to produce an effect was first clearly enunciated 

 by Grotthus (1819, p. 101) and, independently, at a later date, by Draper (1841). 

 It is frequently known as Draper's law. 



Grotthus found, for example, that ferric thiocyanate, which is red, is decolorised by 



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