physics-subatomic-particles

Soon-after the great sucess of Bohr's quantised atomic model, another convincingargument for the Quantum Theory was found by A .Compton while he was studying th escattering of x-rays by matter . X-rays had first been observed by Rtntgen when he wa sexperimenting with a cathode ray tube enclosed in black paper . He noticed some fluorescencein a barium platinocyanide screen nearby, and found that even when variou smaterials were placed between the cathode ray tube and the observation screen, th efluorescence was never radically decreased . He attributed this phenomenon to a ne wradiation, which he named 'x-rrys' . He discovered that x-rays affected photographi cplates, and that they were not easily refracted . For a short time there was controvers yamong scientists about the nature of x-rays, but by 1913 they had been shown to besimilar in nature to light, but with the much shorter wavelength of around 100 pm .The discovery that x-rays were produced whenever fast-moving electrons were stoppe dsoon caused the development of the x-ray tube . In the most primitive of these, electron swere emitted by the bombardment of a cathode with positive ions, as in simple discharg etubes . The newly-produced electrons were then accelerated by the potential differenc ebetween the cathode and the target, which served as an anode . The cathode was concave ,so that only a very small area of the target was bombarded by electrons, thus producin gan almost point source of x-rays . But in 1913, Coolidge made a great improvement on thi sso-called 'cold cathode' tube . He constructed an x-ray tube which used a hot filament _as its source of electrons, which greatly increased the x-ray intensity :obtainable .Using x-rays produced by this new kind of tube, Bragg and others soon initiated suchprinciples as x-ray crystallography and spectrometry .When J .J .Thomson used the comparatively long wavelength x-rays produced by col dcathode tubes to probe into atomic structure, there was no noticeable change in wavelengthwhere the x-rays were scattered by matter . But when later experimenters use dhigher-frequency x-rays, they noticed a considerable change in frequency when thei rx-rays were scattered by some of the lighter elements . In 1923 Compton and Debye foun dthat the only way to explain this increase in wavelength was to assume that x-ray sconsisted of discrete 'packets' of electromagnetic energy, which bounced off individua lparticles in the atoms of the scattering substance . Using this hypothesis, they foun dthat :'h->.,• m cc (1 - cos 9), where is the new wavelength, is the original wavelength ,m,is the rest mass of the electron, h is Planck's constant, c is the velocity o flight in vacuo, and Q is the angle through which x-ray quanta or photons arescattered . This formula was soon adequately borne out by experimental results for al lvalues of the scatter angle less than 150 0 .Ever since the beginning of the nineteenth century, the wave theory of light hadhad the upper hand over the corpuscular theory, and the latter had been almos tcompletely rejected . But suddenly Einstein showed that the two theories were no tmutually exclusive, and that each had its role in physics . In 1923 L .de Brogli erealised that the 'dual' idea of light suggested by Einstein must similarly apply toall other matter, notably electrons . De Broglie knew Einstein's celebrated mass-energ yconversion equation E .- mc z , and, substituting for E in the Planck-Einstein equatio nE- hv, he found that :.X . h /( mv ) ,where m is the mass of an object, v is its velocity, is its ' wavelength', and his Planck's constant . Thus he suggested that any moving particle has a matter wav eassociated with it, and initiated the principle of 'duality' .