DUAL NATURE OF RADIATION AND MATTER || Notes ||

                 1. Introduction

This topic deals with the DUAL NATURE OF RADIATION AND MATTER. 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.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 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 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 ×10^8 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 electron, through his theoretical and experimental investigations on conduction of electricity by gasses, he was awarded the 

Nobel Prize in Physics in 1906. In 1913, the American physicist R. A. Millikan 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.

              2. ELECTRON EMISSION

The metals have free electrons (negatively charged particles) that are responsible for their conductivity. However, the free electrons cannot 

normally escape out of 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 φ 0 and measured in e V (electron volt). 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 e V = 1.602 ×10–19 J. This unit of energy is commonly used in atomic and nuclear physics. The work function (φ 0) depends on the properties of the metal and the nature of its surface. These values are approximate as they are very sensitive to surface impurities. The work function of platinum is the highest (φ 0 = 5.65 e V) while it is the lowest (φ 0  = 2.14 e V) 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.

(i i) Field emission: By applying a very strong electric field (of the order of 10^8 V m–1) to a metal, electrons can be pulled out of the metal, as in a spark plug. 

(i i i) 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.
 

3. PHOTOELECTRIC EFFECT AND WAVE THEORY OF LIGHT

The phenomena of interference, diffraction and polarisation were explained in a natural and satisfactory way by the wave picture of light. According to this picture, light is an electro magnetic wave consisting of electric and magnetic fields with continuous distribution of energy over the region of space over which the wave is extended. Let us now see if this wave picture of light can explain the observations on photoelectric emission given in the previous section. According to the wave picture of light, the free electrons at the surface of the metal (over which the beam of radiation falls) absorb the radiant energy continuously. The greater the intensity of radiation, the greater are the amplitude of electric and magnetic fields. Consequently, the greater the intensity, the greater should be the energy absorbed by each electron. The maximum kinetic energy of the photoelectrons on the surface is then expected to increase with increase in intensity. Also, no 

matter what the frequency of radiation is, a sufficiently intense beam of radiation (over sufficient time) should be able to impart enough energy to the electrons, so that they exceed the minimum energy needed to escape from the metal surface . A threshold frequency, therefore, should not exist. Further, we should note that in the wave picture, the absorption of energy by electron takes place continuously over the entire 

wavefront of the radiation. Since a large number of electrons absorb energy, the energy absorbed per electron per unit time turns out to be small. 

Explicit calculations estimate that it can take hours or more for a single electron to pick up sufficient energy to overcome the work function and come out of the metal. This conclusion is again in striking contrast to observation (i v) that the photoelectric emission is instantaneous. In short, the wave picture is unable to explain the most basic features of photoelectric emission.