dual nature

• Elabonic Emission

  Phenomenon in which electrons are emitted from a material after absorbing external energy
• Types of Elabonic Emission

  • Thermionic
  • Field Emission
  • Photoelectric Emission
• Photoelectric Effect


  • Particle nature of light
  • Light is made up of energy packets called photons
  • Photon is consumed completely when it falls on the surface of a metal
• Photon


  Energy packet of light
• Planck's constant

  6.626 x 10^-34 Js
• Photon energy (Ep)


  Equals the difference between the energy of the photon (hc/λ) and the work function of the metal (Wo)
• Photon does not carry any charge</b>
• Photon is not deflected by electric and magnetic fields
• Possible outcomes of photoelectric effect

  • Ep < Wo: No photoelectric effect
  • Ep = Wo: Minimum energy required for photoelectric effect
  • Ep > Wo: Photoelectric effect occurs, kinetic energy of emitted electron = Ep - Wo
• Metals with smaller work function are more photosensitive
• . The discovery of X-rays by Roentgen and of electron by J. J. Thomson
• cathode rays were discoveredby William Crookes
• 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
• speeds ranging from about 0.1 to 0.2 times the speed of light (3 ×10^8 m/s).
• 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
• Millikan’s experiment established that electric charge is quantised
• millikan oil drop experiment for precise measurement of the charge on an electron
• work function of platinum is the highest (φ0 = 5.65 eV)

  while it is the lowest (φ0 = 2.14 eV) for caesium
• photoelectric emission was discovered by Heinrich Hertz
• Hallwachs and Philipp Lenard investigated the phenomenon of photoelectric
• • Hallwachs undertook the study further and connected a negatively charged zinc plate to an electroscope.
  • He observed that the zinc plate lost its charge when it was illuminated by ultraviolet light.
  • Further, the uncharged zinc plate became positively charged when it was irradiated by ultraviolet light.
  • Positive charge on a positively charged zinc plate was found to be further enhanced when it was illuminated by ultraviolet light.
• metals like zinc, cadmium, magnesium, etc., responded only to ultraviolet ligh
• some alkali metals such as lithium, sodium, potassium, caesium and rubidium were sensitive even to visible light.

  All these photosensitive substances emit electrons when they are illuminated by light
• number of photoelectrons emitted per second is directly proportional to the intensity of incident radiation
• • maximum value of the photoelectric current is called saturation current.
• All the photoelectrons emitted from the metal do not have the same energy
• for a given frequency of the incident radiation, the stopping potential is independent of its intensity
• • the photoelectric emission starts instantaneously without any apparent time lag, even if the incident radiation is very dim.
  • It is now known that emission starts in a time of the order of 10– 9 s or less.
• Albert Einstein proposed a radically new picture of electromagnetic radiation to explain photoelectric effect
• Radiation energy is built up of discrete units – the so called quanta of energy of radiation
• Kmax = hν – φ0 as Einstein photoelectric equation
• Low intensity does not mean delay in emission
• low intesity only determines how many electrons are able to participate in the elementary process (absorption of a light quantum by a single electron) and, therefore, the photoelectric current.
• Millikan performed a series of experiments on photoelectric effect, aimed at disproving Einstein’s photoelectric equation
• Leonard observed that when ultraviolet radiation were allowed to fall on the emitted plate of an evacuated glass tube current flows in circuit
• The particle-like behaviour of light was confirmed by A.H. Compton on scattering of X-rays from electrons
• Einstein was awarded the Nobel Prize for his contribution to theoretical physics and the photoelectric effect.
• Millikan was awarded the Nobel Prize for his work on the elementary charge of electricity and on the photoelectric effect.
• Photons are electrically neutral and are not deflected by electric and magnetic fields.
• Louis Victor de Broglie put forward the bold hypothesis that moving particles of matter should display wave-like properties