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Dual Nature of Radiation and Matter 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 8 to 0.2 times the spe11 of light (3 ×10 m/s). The presently accepted value of e/m is 1.76 × 10 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 smallspeeds.Also,certainmetalswhenheatedtoahightemperaturewere foundtoemitnegativelychargedparticles.Thevalueofe/moftheseparticles was found to be the same as that for cathode ray particles. These observations thus established that all these particles, although produced underdifferentconditions,wereidenticalinnature.J.J.Thomson,in1897, 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 (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 –19 charge, 1.602 × 10 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. 11.2 E LECTRON E MISSION We know that metals have freeelectrons (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 outofthemetal,themetalsurfaceacquiresapositivecharge andpullsthe 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 comeoutofthemetalsurfaceonlyifithasgotsufficientenergytoovercome 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 0and measured in eV (electron volt). One electron volt is the energy gained by an electron when it has been acceleratedby a potentialdifference of1volt,so that 1 eV = 1.602 ×10 –19J. This unit of energy is commonly used in atomic and nuclear physics. The work function ( ) 0epends on the properties of the metal and the nature of its surface. Theminimumenergyrequiredfortheelectronemissionfromthemetal 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. 275 Reprint 2026-27