CHAPTER 4 



Electromagnetic Radiations 

 and Matter 



The next thing is striking: through the black carton container, which lets 

 through no visible or ultraviolet rays of the sun, nor the electric arc light, an 

 agent (X) goes through which has the property that it can produce a vivid 

 fluorescence .... 



We soon found that the agent penetrates all bodies, but to a very different 

 degree. (W. C. Roentgen, Annalen der Physik und Chemie, 64, 1 

 (1898).) 



INTRODUCTION 



Within fifteen years, just before the turn of the century, complacent classi- 

 cal physics received three rude shocks. The first was Julius Plucker's de- 

 scription (circa 1890) of the electrical discharges which take place in gases 

 under low pressure and high voltage (the embryo of the "neon" sign). The 

 second was Henri Becquerel's discovery of natural radioactivity in 1895; and 

 the third was Wilhelm Roentgen's discovery of X rays, reported in 1898. In 

 the years since then, the three discoveries have collectively engendered in- 

 tense investigation of: (1) the structure of molecules, atoms and nuclei; 

 (2) arrangements of molecules in crystals and other, less well-defined molec- 

 ular arrays; (3) the electromagnetic spectrum, from X rays through visible 

 to infrared radiation; and (4) the interactions — and in fact interconversion! 

 — of electromagnetic energy and matter. In this chapter a review is given of 

 those facts and theories which are useful to an understanding of the bio- 

 physics of the interactions of electromagnetic radiation and living matter. 



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