Quantum Mechanics Photoelectric Effect

Lesson Aims State the main observations of Millikan’s experiment Explain the failure of classical wave theory to explain the observations Describe Einstein’s 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

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

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 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)

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

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."

Millikan’s Experiment Millikans experiment was able to find the maximum kinetic energy of ejected electrons

Millikan’s Results The results allow threshold frequencies to be calculated for different metals