CHAPTER I 



LIGHT AND THERMAL RADIATIONS 



How can light be defined physically, and without ambiguity, 

 in quality and in quantity? Why is it not sufficient to say, for 

 example that a certain plant was cultivated in the laboratory 

 under an illumination of 1,000 lux of yellow light? What are 

 the imprecisions, for there are several, in that statement? 

 We cannot answer the question without explaining some 

 physical characteristics of Ught. 



One of the fundamental quantities of physics is energy. 

 The principle of the conservation of energy has always been 

 confirmed. It has survived all the evolutions of physical 

 thought, from relativity to wave mechanics. It has even 

 acquired in those theories a still deeper significance. Now, 

 light is a form of energy and sunlight brings us an enormous 

 quantity of it — nearly the whole of that which is available 

 to us. 



Light-waves 



Light consists of waves, of an electromagnetic nature, 

 which are propagated as well, and even better, in a vacuum 

 than in the air or transparent media. Of the same nature are 

 the ultra-violet and infra-red rays, the X-rays used in medicine, 

 the y-rays of radioactive substances, and the long or short 

 Hertzian waves used in radio transmission. 



We know that the Hertzian waves of telegraphy and of 

 wireless telephony are characterized by their wave-length. 

 The nature of Ught is specified in exactly the same way. The 

 apparatus which separates its various ^wave-lengths is called 

 a monochromator. It generally contains one or more prisms 

 and isolates pure, or monochromatic, waves. The energy of 



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