Some Contemporary Advances in Physics-III

Abstract

ELECTROMAGNETIC waves of every frequency from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> exist; they can be generated and perceived; their frequencies in nearly every instance can be measured; their actions and reactions with matter can be studied. This brief statement is the synthesis of a great multitude of inventions, experiments and observations upon phenomena of extraordinary diversity and variety. When 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 that the effect of the sunbeam on the mercury column did not cease when it passed beyond the red edge of the fan, he proved that the boundary 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 Ritter found that the power of the violet rays to affect salts of silver was shared by invisible rays beyond the violet edge of the beam. 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 ultraviolet rays were found to have some of these properties, while the outstanding discordances were explained away by Maxwell's successors. Hertz and many others built apparatus for producing Maxwell's waves with frequencies far below those of light, and apparatus for detecting them, with consequences known to everyone. Years after X-rays and gamma-rays were discovered emanating from discharge-tubes and disintegrating atoms, Laue proved that these too are waves, lying beyond the visible spectrum in the range of high frequencies. Radiations emerging from collapsing atoms and radiations diverging from wireless towers; waves conveying the solar heat and waves carrying the voice; rays which disrupt atoms by extracting their electrons, rays which alter atoms by rearranging their electrons, rays which almost ignore atoms altogether, were successively discovered or created; and all these radiations were brought into one class, and identified with light.