The Discovery of the Quantum

The Discovery of the Quantum
By the late 1800's much of science had been well understood. Naturally, scientists of the time generally believed that most major problems in physics had been solved and that all that was left was "clean-up" research. Nothing could have been further from the truth. There were three little known experiments at the time that defied explanation by traditional science. These experiments were:


  • Black-body radiation
  • Photoelectric Effect
  • Emission Spectrum of Hydrogen
  • The eventual explanation of these experiments led to a revoltion in science and how science viewed atoms and their interaction with light.

    Black-body Radiation

    If a hollow black-body is heated to a temperature T it will emit light. The wavelength dependence of the energy of this light can be recorded. The resulting curve is the black-body radiation curve.

    Science could not explain the wavelength dependence of the curve until Max Planck focused on the problem around 1900. Max Planck was an expert in the science of thermodynamics and he used the tools of thermodynamics to try and explain BB radiation. However, after several failures he almost gave up. He made one last attempt. He decided to throw away the ideas of classical science and make a bold assumption: He assumed that the atoms in the black-body could not absorb just any energy but could only absorb or emit energy in packets he called "quanta". With this assumption he quickly derived a formula that exactly reproduced the black-body radiation spectrum. Since Planck was the first scientist to suggest "quanta" he is today considered the father of quantum theory.


    Photo Electric Effect

    The world of science did not pay too much attention to Planck's work because his assumption of quanta was ridiculous to their minds. In 1905 Einstein investigated the phenomenon known as the photoelectric effect. The photoelectric effect is simply the ability of some metals such as potassium to eject electrons when irradiated by light:

    Traditional physics predicted that the energy of the ejected electron would depend upon the intensity of light and independent of the wavelength. However, experimentally the opposite was observed.

    Einstein knew of the work of Planck and decided that it might have application to the photoelectric effect. Einstein suggested that light, although traditionally viewed as a wave could instead be viewed as packets of light he called "photons". These light packets each had energy which was given by

    E(photon) = h n

    With this view Einstein was able to completely explain the photoelectric effect. Einstein clearly understood that his photons were similar to Plancks' quanta.

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    C101 Class Notes
    Prof. N. De Leon