Dual nature of radiation


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Dual nature of radiation

  3. 3.  Wave–particle duality postulates that all particles exhibit both wave and particle properties. A central concept of quantum mechanics, this duality addresses the inability of classical concepts like "particle" and "wave" to fully describe the behavior of quantum-scale objects. Standard interpretations of quantum mechanics explain this paradox as a fundamental property of the Universe, while alternative interpretations explain the duality as an emergent, second-order consequence of various limitations of the observer. This treatment focuses on explaining the behavior from the perspective of the widely used Copenhagen interpretation, in which wave–particle duality is one aspect of the concept of complementarity, that a phenomenon can be viewed in one way or in another, but not both
  4. 4.  The idea of duality originated in a debate over the nature of light and matter that dates back to the 17th century, when competing theories of light were proposed by Christiaan Huygens and Isaac Newton: light was thought either to consist of waves (Huygens) or of particles (Newton). Through the work of Max Planck, Albert Einstein, Louis de Broglie,Arthur Compton, Niels Bohr, and many others, current scientific theory holds that all particles also have a wave nature (and vice versa).[1] This phenomenon has been verified not only for elementary particles, but also for compound particles like atoms and even molecules. For macroscopic particles, because of their extremely small wavelengths, wave properties usually cannot be detected
  5. 5.  Aristotle was one of the first to publicly hypothesize about the nature of light, proposing that light is a disturbance in the element air (that is, it is a wave-like phenomenon). On the other hand, Democritus—the original atomist—argued that all things in the universe, including light, are composed of indivisible sub- components (light being some form of solar atom).[3] At the beginning of the 11th Century, the Arabic scientist Alhazen wrote the first comprehensive treatise on optics; describing refraction, reflection, and the operation of a pinhole lens via rays of light traveling from the point of emission to the eye. He asserted that these rays were composed of particles of light. In 1630, René Descartes popularized and accredited in the West the opposing wave description in his treatise on light, showing that the behavior of light could be re-created by modeling wave-like disturbances in a universal medium ("plenum"). Beginning in 1670 and progressing over three decades, Isaac Newton developed and championed his corpuscular hypothesis, arguing that the perfectly straight lines of reflection demonstrated lights particle nature; only particles could travel in such straight lines. He explained refraction by positing that particles of light accelerated laterally upon entering a denser medium. Around the same time, Newtons contemporaries Robert Hooke and Christian Huygens—and later Augustin-Jean Fresnel—mathematically refined the wave viewpoint, showing that if light traveled at different speeds in different media (such as water and air), refraction could be easily explained as the medium-dependent propagation of light waves. The resulting Huygens–Fresnel principle was extremely successful at reproducing lights behavior and, subsequently supported byThomas Youngs discovery of double-slit interference, was the beginning of the end for the particle light camp
  7. 7.  The liberation of electrons from the surface of a metal is known as Electron Emission. If a piece of metal is investigated at room temperature, the random motion of the electrons will be shown in Fig. However, these electrons are free to the extent that they may transfer from one atom to another within the metal but they cannot leave the metal surface to provide electron mission. It is because the free electrons that start at the surface of metal find behind them positive nuclei pulling them back and none pulling forward. Thus at the surface of the metal , a free electron encounters forces that prevent it to leave the metal. In other words, the metallic surface offer a barrier to free electrons, their kinetic energy increases and is known as surface barrier. However, if sufficient energy is given to the free electrons, their kinetic energy increases and thus the electrons will cross over the surface barrier to leave the metal.
  8. 8.  Work function (W0): The minimum energy required by an electron to just escape (i.e. with zero velocity) from metals surface is called Work function (W0) of the metal. The work function of pure metals varies (roughly) from 2eV to 6eV. Its value depends upon the nature of the metal, its purity and the conditions of the surface. We selected those metals for electron emission which have low work function.
  9. 9.  The electron emission from the surface of a metal is possible only if sufficient addition energy (equal to work function of the sources such as heat energy, energy stored in electric field, light energy or kinetic energy of the electric charges bombarding the metal surface. Accordingly; there are following four principal method of obtaining electron emission from (I) Thermionic emission: In this method, the metal is heated to a sufficient temperature (about 2500oC) to enable the free electrons to leave the metal surface. The number of electrons emitted depends upon the temperature. The higher the temperature, the greater is the emission of electrons. This type of emission is employed in vacuu (II) Field emission: In this method, a strong electric field (i.e. a high positive voltage) is applied at the metal surface which pulls the free electrons out of the metal because of the attraction of positive field. The strong the electric field, the greater is the electron emission.m tubes.the surface of a metal:
  10. 10.  (III) Photoelectric emission: In this method, the energy of light falling upon the metal surface is transferred to the free electrons within the metal to enable them to leave the surface. The greater the intensity of light beam falling on the metal surface, the greater is the photoelectric emission. Photoelectric emission is utilized in photo tubes which from the basis of television and sound films. (IV) Secondary emission: In this method, a high velocity beam of electrons or other out. The intensity of secondary emission depends upon the emitter material, mass and energy of bombarding particles.m the basis of television and sound films.
  11. 11.  In the photoelectric effect, electrons are emitted from matter (metals and non-metallic solids, liquids or gases) as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength and high frequency, such as ultraviolet radiation. Electrons emitted in this manner may be referred to as photoelectrons.[1][2] First observed by Heinrich Hertz in 1887,[] the phenomenon is also known as the Hertz effect,[ although the latter term has fallen out of general use. Hertz observed and then showed that electrodes illuminated with ultraviolet light create electric sparks more easily. The photoelectric effect requires photons with energies from a few electronvolts to over 1 MeV in high atomic number elements. Study of the photoelectric effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of wave–particle duality.[1] Other phenomena where light affects the movement of electric charges include the photoconductive effect (also known as photoconductivity or photoresistivity), the photovoltaic effect, and the photoelectrochemical effect. It also led to Max Plancks discovery of quanta (e=hv) which links frequency with photon energy. Quanta is also known asPlanck constant.
  12. 12.  Emission mechanism The photons of a light beam have a characteristic energy proportional to the frequency of the light. In the photoemission process, if an electron within some material absorbs the energy of one photon and acquires more energy than the work function (the electron binding energy) of the material, it is ejected. If the photon energy is too low, the electron is unable to escape the material. Increasing the intensity of the light beam increases the number of photons in the light beam, and thus increases the number of electrons excited, but does not increase the energy that each electron possesses. The energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy or frequency of the individual photons. It is an interaction between the incident photon and the outermost electron. Electrons can absorb energy from photons when irradiated, but they usually follow an "all or nothing" principle. All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or else the energy is re-emitted. If the photon energy is absorbed, some of the energy liberates the electron from the atom, and the rest contributes to the electrons kinetic energy as a free particle
  13. 13.  The theory of the photoelectric effect must explain the experimental observations of the emission of electrons from an illuminated metal surface. For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons are emitted. This frequency is called the threshold frequency. Increasing the frequency of the incident beam, keeping the number of incident photons fixed (this would result in a proportionate increase in energy) increases the maximum kinetic energy of the photoelectrons emitted. Thus the stopping voltage increases. The number of electrons also changes because the probability that each photon results in an emitted electron is a function of photon energy. If the intensity of the incident radiation is increased, there is no effect on the kinetic energies of the photoelectrons. Above the threshold frequency, the maximum kinetic energy of the emitted photoelectron depends on the frequency of the incident light, but is independent of the intensity of the incident light so long as the latter is not too high [9] For a given metal and frequency of incident radiation, the rate at which photoelectrons are ejected is directly proportional to the intensity of the incident light. Increase in intensity of incident beam (keeping the frequency fixed) increases the magnitude of the photoelectric current, though stopping voltage remains the same. The time lag between the incidence of radiation and the emission of a photoelectron is very small, less than 10−9 second. The direction of distribution of emitted electrons peaks in the direction of polarization (the direction of the electric field) of the incident light, if it is linearly polarized
  14. 14.  Stopping potential The relation between current and applied voltage illustrates the nature of the photoelectric effect. For discussion, a light source illuminates a plate P, and another plate electrode Q collects any emitted electrons. We vary the potential between P and Q and measure the current flowing in the external circuit between the two plates. If the frequency and the intensity of the incident radiation are fixed, the photoelectric current increases gradually with an increase in positive potential on collector electrode until all the photoelectrons emitted are collected. The photoelectric current attains a saturation value and does not increase further for any increase in the positive potential. The saturation current depends on the intensity of illumination, but not its wavelength. If we apply a negative potential to plate Q with respect to plate P and gradually increase it, the photoelectric current decreases until it is zero, at a certain negative potential on plate Q. The minimum negative potential given to plate Q at which the photoelectric current becomes zero is called stopping potential or cut off potential.[13] i. For the given frequency of incident radiation, the stopping potential is independent of its intensity. ii. For a given frequency of the incident radiation, the stopping potential Vo is related to the maximum kinetic energy of the photoelectron that is just stopped from reaching plate Q. If is the mass and is the maximum velocity of photoelectron emitted
  15. 15.  EINSTEINS PHOTOELECTRIC EQUATION According to Planks quantum theory, light is emitted from a source in the forms of bundles of energy called photons. Energy of each photon is . Einstein made use of this theory to explain how photo electric emission takes place. According to Einstein, when photons of energy fall on a metal surface, they transfer their energy to the electrons of metal. When the energy of photon is larger than the minimum energy required by the electrons to leave the metal surface, the emission of electrons take place instantaneously. He proposed that an electron absorbs one whole photon or none. The chance that an electron may absorb more then one electron is negligible because the number of photons is much lower than the electron. After absorbing the photon, an electron either leaves the surface or dissipates its energy within the metal in such a short interval that it has almost no chance to absorb second photon. An increase in intensity of light source simply increases the number of photon and the number of photo electrons but no increase in the energy of photo electron. However, increase in frequency increases the energy of photons and photo electrons.
  16. 16.  Light as a particle The only thing that interferes with my learning is my education. -- Albert Einstein Radioactivity is random, but do the laws of physics exhibit randomness in other contexts besides radioactivity? Yes. Radioactive decay was just a good playpen to get us started with concepts of randomness, because all atoms of a given isotope are identical. By stocking the playpen with an unlimited supply of identical atom-toys, nature helped us to realize that their future behavior could be different regardless of their original identicality. We are now ready to leave the playpen, and see how randomness fits into the structure of physics at the most fundamental level. The laws of physics describe light and matter, and the quantum revolution rewrote both descriptions. Radioactivity was a good example of matters behaving in a way that was inconsistent with classical physics, but if we want to get under the hood and understand how nonclassical things happen, it will be easier to focus on light rather than matter. A radioactive atom such as uranium-235 is after all an extremely complex system, consisting of 92 protons, 143 neutrons, and 92 electrons. Light, however, can be a simple sine wave. However successful the classical wave theory of light had been --- allowing
  17. 17. The Davisson–Germer experiment was a physics experiment conducted by American physicists Clinton Davisson and Lester Germer in 1927, which confirmed the de Broglie hypothesis. This hypothesis advanced by Louis de Broglie in 1924 says that particles of matter such as electrons have wave like properties. The experiment not only played a major role in verifying the de Broglie hypothesis and demonstrated the wave-particle duality, but also was an important historical development in the establishment of quantum mechanics and of the Schrödinger equation
  18. 18.  Davisson and Germers actual objective was to study the surface of a piece of nickel by directing a beam of electrons at the surface and observing how many electrons bounced off at various angles. They expected that for electrons even the smoothest crystal surface would be too rough and so the electron beam would experience diffuse reflection.[5] The experiment consisted of firing an electron beam from an electron gun directed to a piece of nickel crystal at normal incidence (i.e. perpendicular to the surface of the crystal). The experiment included an electron gun consisting of a heated filament that released thermally excited electrons, which were then accelerated through a potential difference giving them a certain amount of kinetic energy towards the nickel crystal. To avoid collisions of the electrons with other molecules on their way towards the surface, the experiment was conducted in a vacuum chamber. To measure the number of electrons that were scattered at different angles, an electron detector that could be moved on an arc path about the crystal was used. The detector was designed to accept only elastically scattered electrons. During the experiment an accident occurred and air entered the chamber, producing an oxide film on the nickel surface. To remove the oxide, Davisson and Germer heated the specimen in a high temperature oven, not knowing that this affected the formerly polycrystalline structure of the nickel to form large single crystal areas with crystal planes continuous over the width of the electron beam.[5] When they started the experiment again and the electrons hit the surface, they were scattered by atoms which originated from crystal planes inside the nickel crystal. As Max von Laue proved in 1912 the crystal structure serves as a type of three dimensional diffraction grating. The angles of maxim