Quantum Mechanics

1
Quantum Mechanics

• Photoelectric Effect

2
Lesson Aims

• State the main observations of Millikans
  experiment
• Explain the failure of classical wave theory to
  explain the observations
• Describe Einsteins explanation
• Define work function and threshold frequency
• Solve problems using photoelectric effect
• Explain the significance of contributions from
  Newton, Huygen and Young to the quantum theory of
  photoelectric emission

3
Review

• As a material is heated electrons in the material
  gain thermal energy
• Electrons in the material the oscillate with
  greater frequency than before
• The theory predicted that the wavelength of
  emission was proportional temperature
• However, at short wavelength this approximation
  fails and it is not possible to produce UV, X and
  gamma rays by heating a material. Ultraviolet
  Catastrophe!
• Instead, it was postulated that energy radiated
  by a black body (material) was quantised

4
History

• Wien received a Nobel Prize (1911) for his work
  on heat radiation (1893)
• Planck received a Nobel Prize (1918) for his
  quantum contribution to this field (1894)
• Einstein produced the Annus Mirabilis Papers in
  1905. Included in which was "On a Heuristic
  Viewpoint Concerning the Production and
  Transformation of Light"
• Einstein was motivated by Planck and made
  assumptions that light can only be absorbed or
  emitted in discrete amounts. This provided an
  explanation the photoelectric effect and Einstein
  received the Nobel Prize (1921)
• Millikan of oil-drop fame, for which he
  calculated the charge of the electron accurately
  (1913), and won a Nobel Prize (1923) for his
  efforts, did not agree with a recent paper by
  Einstein (he also fudged results for his seminal
  experiment)

5
Photoelectric Effect

• Henri Becquerel's father exposed an electrode to
  light and discovered the photoelectric effect in
  1839
• By 1887, Hertz made observations of the
  production and reception of electromagnetic waves
• By 1899, JJ Thomson was able to study UV rays in
  CRTs and noted that a subatomic corpuscle was
  emitted from a cathode which subsequently
  produced a current
• In 1902, Von Lenard performed experiments in
  which he showed that electrons could be emitted
  from metals by the absorption of light. He also
  proved that this phenomenon was dependent upon
  the frequency of the incident light
• The emission of electrons by the absorption of
  light is known as the photoelectric effect

6
Photoelectric Effect

• Millikan spent a decade perfecting an experiment
  to disprove Einstein's explanation of the
  photoelectric effect
• He was actually the first person to confirm
  Einstein's theory, although he could never fully
  admit it
• In his 1950 autobiography he declared that his
  work "scarcely permits of any other
  interpretation than that which Einstein had
  originally suggested, namely that of the
  semi-corpuscular or photon theory of light
  itself."

7
Millikans Experiment

• Millikans experiment was able to find the maximum
  kinetic energy of ejected electrons

8
Millikans Results

• The results allow threshold frequencies to be
  calculated for different metals