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Some Contemporary Advances in Physics -III

01 April 1924

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L E C T R O M A G N E T I C waves of every frequency from 104 to 1020 exist; they can be generated and perceived; their frequencies in nearly every instance can be measured; their actions and reactions with m a t t e r can be studied. T h i s brief s t a t e m e n t is t h e synthesis of a great m u l t i t u d e of inventions, experiments and observations upon phenomena of extraordinary diversity and variety. W h e n Herschel in 1800 carried a thermometer across the fan-shaped beam of colored light into which a sunbeam was resolved by a prism, and observed t h a t the effect of t h e sunbeam on the mercury column did not cease when it passed beyond the red edge of t h e fan, he proved t h a t the b o u n d a r y of the spectrum beyond the red is imposed by the limitations of the eye and not by a deficiency of rays. Almost at the same time R i t t e r found t h a t the power of the violet rays to affect salts of silver was shared by invisible rays beyond the violet edge of the b e a m . Maxwell developed the notion of electromagnetic waves from his theory of electricity and magnetism, and described some of the properties they should have; and the light-waves and the infra-red and ultra-violet rays were found to have some of these properties, while t h e o u t s t a n d i n g discordances were explained a w a y by Maxwell's successors. H e r t z and m a n y others built a p p a r a t u s for producing Maxwell's waves with frequencies far below those of light, and app a r a t u s for detecting them, with consequences known to everyone.