Dual Nature of Radiation and Matter

Objectives

---

• the phenomenon of electron emission and its types • the observations of Hertz, Hallwachs and Lenard • photoelectric effect and its laws • the concept of quantization of energy • photo cell and its applications • particle nature of radiation • the wave nature of matter • de Broglie equation and de Broglie wavelength of electron • the construction and working of electron microscope • Davisson and Germer experiment • X-rays and its production • X-rays spectra and its types

Summary

---

„ Particle is a material object which is considered as a tiny concentration of matter (localized in space and time) whereas wave is a broad distribution of energy (not localized in space and time). „ The liberation of electrons from any surface of a substance is called electron emission. „ The minimum energy needed for an electron to escape from the metal surface is called work function of that metal. „ 1 eV is equal to 1.602 × 10–19 J. „ The emission of electrons by supplying thermal energy is known as thermionic emission. „ Electric field emission occurs when a very strong electric field is applied across the metal. „ The emission of electrons due to irradiation of light is called photoelectric emission. „ Secondary emission is the process in which electrons are emitted due to the bombardment of fast moving electrons. „ The photoelectric current (i.e. the number of electrons emitted per second) is directly proportional to the intensity of the incident light. „ Stopping potential is that the value of the negative (retarding) potential given to the collecting electrode A which is just sufficient to stop the most energetic photoelectrons emitted and make the photocurrent zero. „ The stopping potential is independent of intensity of the incident light. „ Maximum kinetic energy of the photoelectrons is independent of intensity of the incident light. „ For a given surface, the emission of photoelectrons takes place only if the frequency of incident light is greater than a certain minimum frequency called the threshold frequency. „ According to Planck, a matter is composed of a large number of oscillating particles (atoms) which vibrate with different frequencies. „ According to Einstein, the energy in light is not spread out over wavefronts but is concentrated in small packets or energy quanta. „ The individual light quantum of definite energy and momentum is called photon. „ Light behaves as a wave during its propagation and behaves as a particle during its interaction with matter. „ Photo electric cell or photo cell is a device which converts light energy into electrical energy. „ According to de Broglie hypothesis, all material particles like electrons, protons, neutrons in motion possess wave nature. These waves associated with them are called de Broglie waves or matter waves. „ Wave nature of the electron is used in the construction of electron microscope. „ Louis de Broglie hypothesis of matter waves was experimentally confirmed by Clinton Davisson and Lester Germer in 1927. „ Whenever fast moving electrons fall on the materials, a highly penetrating radiations, namely x-rays, are emitted. „ Continuous x-ray spectrum consists of radiations of all possible wavelengths with a certain minimum wavelength λ0 . „ Characteristic x-ray spectra show some narrow peaks at some well–defined wavelengths when the target is hit by fast electrons.