NCERT Book Class 12 Physics Chapter 11 Dual Nature of Radiation and Matter

Physics

Chapter Eleven

DUAL NATURE OF
RADIATION AND
MATTER

11.1 INTRODUCTION
The Maxwell’s equations of electromagnetism and Hertz experiments on
the generation and detection of electromagnetic waves in 1887 strongly
established the wave nature of light. Towards the same period at the end
of 19th century, experimental investigations on conduction of electricity
(electric discharge) through gases at low pressure in a discharge tube led
to many historic discoveries. The discovery of X-rays by Roentgen in 1895,
and of electron by J. J. Thomson in 1897, were important milestones in
the understanding of atomic structure. It was found that at sufficiently
low pressure of about 0.001 mm of mercury column, a discharge took
place between the two electrodes on applying the electric field to the gas
in the discharge tube. A fluorescent glow appeared on the glass opposite
to cathode. The colour of glow of the glass depended on the type of glass,
it being yellowish-green for soda glass. The cause of this fluorescence
was attributed to the radiation which appeared to be coming from the
cathode. These cathode rays were discovered, in 1870, by William
Crookes who later, in 1879, suggested that these rays consisted of streams
of fast moving negatively charged particles. The British physicist
J. J. Thomson (1856 -1940) confirmed this hypothesis. By applying
mutually perpendicular electric and magnetic fields across the discharge
274 tube, J. J. Thomson was the first to determine experimentally the speed