Newton: 1865- Corpuscular Theory- Radiation Pressure
This theory cannot explain refraction, diffraction and interference, which require an understanding of the wave theory of light of Christiaan Huygens.
In 1637 Descartes gave the corpuscular model of light and derived Snell’s law
In 1678, the Dutch physicist Christiaan Huygens put forward the wave theory of light
Thomas Young performed his famous interference experiment in 1801.
With the publication of “A Dynamical Theory of the Electromagnetic Field” in 1865, Maxwell demonstrated that electric and magnetic fields travel through space as waves moving at the speed of light
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
experimental investigations on conduction of electricity (electric discharge) through gases at low pressure in a discharge tube
The discovery of X-rays by Roentgen in 1895
Discovery of electron by J. J. Thomson in 1897
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)
In 1887, it was found that certain metals, when irradiated by ultraviolet light, emitted negatively charged particles having small speeds
J. J. Thomson, in 1897, named these particles as electrons, and suggested that they were fundamental, universal constituents of matter. 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 quantised. From the values of charge (e) and specific charge (e/m), the mass (m) of the electron could be determined.
ELECTRON EMISSION
Work Function (φ0 ):– 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
One electron volt is the energy gained by an electron when it has been accelerated by a potential difference of 1 volt, so that 1 eV = 1.602 ×10 –19 J.
work function (φ0 ) depends on the properties of the metal and the nature of its surface. they are very sensitive to surface impurities.
work function of platinum is the highest (φ0 = 5.65 eV) while it is the lowest (φ0 = 2.14 eV) for caesium
The minimum energy required for the electron emission from the metal surface can be supplied to the free electrons by any one of the following physical processes
(i) Thermionic emission: By suitably heating, sufficient thermal energy can be imparted to the free electrons to enable them to come out of the metal.
(ii) Field emission: By applying a very strong electric field (of the order of 108 V m–1) to a metal, electrons can be pulled out of the metal, as in a spark plug.
(iii) Photoelectric emission: When light of suitable frequency illuminates a metal surface, electrons are emitted from the metal surface. These photo(light)-generated electrons are called photoelectrons.
PHOTOELECTRIC EFFECT
Hertz’s observations
The phenomenon of photoelectric emission was discovered in 1887 by Heinrich Hertz (1857-1894). Hertz observed that high voltage sparks across the detector loop were enhanced when the emitter plate was illuminated by ultraviolet light from an arc lam. When light falls on a metal surface, some electrons near the surface absorb enough energy from the incident radiation to overcome the attraction of the positive ions in the material of the surface. After gaining sufficient energy from the incident light, the electrons escape from the surface of the metal into the surrounding space.
Hallwachs’ and Lenard’s observations
Wilhelm Hallwachs and Philipp Lenard investigated the phenomenon of photoelectric emission in detail during 1886-1902. Lenard (1862-1947) observed that when ultraviolet radiations were allowed to fall on the emitter plate of an evacuated glass tube enclosing two electrodes (metal plates), current flows in the circuit
As soon as the ultraviolet radiations were stopped, the current flow also stopped .