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Physics Notes Class 12 Chapter 11 Dual Nature of Radiation PDF Free Download
Dual Nature of Radiation
INTRODUCTION
Maxwell’s equations of electromagnetism and Hertz’s 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 the 19th century, experimental investigations on the 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 electrons by J. J. Thomson in 1897, were important milestones in the understanding of atomic structure.
It was found that at a 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 color of the 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 tube, J. J. Thomson was the first to determine experimentally the speed and the specific charge [charge to mass ratio (e/m )] of the cathode ray particles.
They were found to travel with speeds ranging from about 0.1 to 0.2 times the speed of light (3 ×108 m/s).
The presently accepted value of e/m is 1.76 × 1011 C/kg. Further, the value of e/m was found to be independent of the nature of the material/metal used as the cathode (emitter), or the gas introduced in the discharge tube.
This observation suggested the universality of the cathode ray particles. Around the same time, in 1887, it was found that certain metals, when irradiated by ultraviolet light, emitted negatively charged particles having small speeds.
Also, certain metals when heated to a high temperature were found to emit negatively charged particles.
The value of e/m of these particles was found to be the same as that for cathode ray particles.
These observations thus established that all these particles, although produced under different conditions, were identical in nature.
J. J. Thomson, in 1897, named these particles as electrons and suggested that they were fundamental, universal constituents of matter.
For his epoch-making discovery of electrons, through his theoretical and experimental investigations on the conduction of electricity by gasses, he was awarded the Nobel Prize in Physics in 1906.
In 1913, the American physicist R. A. Millikan (1868-1953) performed the pioneering oil-drop experiment for the precise measurement of the charge on an electron.
He found that the charge on an oil droplet was always an integral multiple of an elementary charge, 1.602 × 10 –19 C. Millikan’s experiment established that electric charge is quantized. From the values of the charge
(e) and specific charge (e/m ), the mass (m) of the electron could be determined.
ELECTRON EMISSION
We know that metals have free electrons (negatively charged particles) that are responsible for their conductivity.
However, the free electrons cannot normally escape from the metal surface. If an electron attempts to come out of the metal, the metal surface acquires a positive charge and pulls the electron back to the metal.
The free electron is thus held inside the metal surface by the attractive forces of the ions. Consequently, the electron can come out of the metal surface only if it has got sufficient energy to overcome the attractive pull.
A certain minimum amount of energy is required to be given to an electron to pull it out from the surface of the metal.
This minimum energy required by an electron to escape from the metal surface is called the work function of the metal.
It is generally denoted by φ0 and measured in eV (electron volt).
| Author | – |
| Language | English |
| No. of Pages | 6 |
| PDF Size | 1 MB |
| Category | Physics |
| Source/Credits | drive.google.com |
Physics Notes Class 12 Chapter 11 Dual Nature of Radiation PDF Free Download