Arthur Holly Compton’s explanation for the change in wavelength of X rays scattered from matter provided an important confirmation of the quantum theory of radiation.
The importance of the phenomenon known as the Compton effect to the fields of chemistry and physics can be appreciated only within the context of early twentieth century science. By the end of the nineteenth century, physical scientists were experiencing a general feeling of complacency. All material substances were known to be composed of molecules, which were understood to be specific combinations of atoms of the ninety or so elemental substances found in nature. The research of Joseph John Thomson indicated clearly that the negatively charged electron is a constituent of all atoms, which, because they are all neutral, had to have some type of positive particle also. Ernest Rutherford’s famous experiment in which he scattered alpha particles from a thin film of gold provided evidence for his theory of atomic structure. Rutherford’s atom was composed of a tiny, but massive, positive nucleus surrounded by space occupied by tiny electrons. Chemists and atomic physicists believed that they had a fairly clear picture of the structure of the material world.
Arthur Holly Compton.
Physicists were enamored of the laws of motion of Sir Isaac Newton,
One of the clear distinctions made by scientists by the end of the nineteenth century was the classification of natural phenomena as either particle or wave phenomena. Particles were thought of as bundles of matter possessing mass, which determines the way in which the particles respond to applied force. In addition to mass, two fundamental properties were associated with particles because of their motion. These are momentum and kinetic energy, both of which are conserved in the absence of interaction with some outside force. A particle of mass m and velocity v has kinetic energy equal to mv2/2 and momentum equal to mv. During a collision (any interaction in which the particles exert forces on one another) between two particles, momentum is always conserved. In some collisions, known as elastic collisions, kinetic energy is conserved also. Conservation of kinetic energy and momentum requires only that the total of each quantity is the same after the collision as before. The particles involved may exchange all or some of their momentum and kinetic energy with one another. Moving particles, then, may be seen as a means of transporting energy from one place to another.
Another means of transporting energy is the wave; familiar examples are water waves, sound waves, and electromagnetic waves such as light. All waves are characterized by wavelength lambda (distance between two identical points on adjacent waves) and frequency v (number of waves per unit time). Waves can be unambiguously identified by their ability to interfere with each other. Whenever two harmonic waves of equal amplitude intersect at some point in a medium, the net effect is the sum of the two waves. If the waves are always in phase at the intersection point, the net wave is twice that of either wave; but, if they are out of phase, they cancel. Two wave systems interfering in a medium give a pattern of amplitudes that is constant in time. Whenever a wave moves around some object or passes through a small opening, an interference pattern between waves coming from different points near the edge of the barrier is set up. This is known as a diffraction pattern. Observation of interference and diffraction patterns is regarded as a confirming test of the wave nature of a phenomenon. There was no doubt in the minds of early twentieth century physicists that light was a wave phenomenon. Diffraction and interference patterns had been observed and used in wavelength measurements.
X rays were discovered by Wilhelm Conrad Röntgen
It was at this point that Arthur Holly Compton investigated this secondary type of scattered radiation. His explanation helped to change the direction of physics. Compton’s career as a research physicist began in 1919 when, as a Fellow of the National Research Council, he studied at Rutherford’s laboratory in Cambridge. He studied the scattering and absorption of gamma rays
After his year at Cambridge, Compton moved to Washington University, where he intended to extend his gamma-scattering experiments into the X-ray region. Using a Bragg crystal spectrometer, he was able to analyze the scattered and primary radiation with great precision. Compton used monochromatic X rays from a molybdenum source, scattered from a target of graphite (a form of carbon), and found that the scattered rays contained radiation that had the same wavelength as the incident radiation as well as radiation of a longer wavelength. The wavelength of this second type of scattered radiation varied in a systematic way with the scattering angle.
This change in wavelength could not be explained in terms of the classical theories of electrodynamics. In 1922, after all attempts to use classical explanations had failed, Compton arrived at his revolutionary quantum theory for the interaction. He treated the process as a collision between a free electron and an X-ray quantum having kinetic energy and momentum. Applying the laws of conservation to the collision, Compton was able to derive the equations for the Compton effect in the form in which they are used today and found exact agreement with his data. The kinetic energy and the momentum of the scattered photon were decreased by an amount equal exactly to that acquired by the electron, which then recoiled. When Compton first proposed his explanation, there was no experimental evidence for the recoil electron, but this evidence was provided shortly afterward by Charles Thomson Rees Wilson, who observed tracks in the cloud chamber, which could be explained in terms of Compton’s theory.
The Compton effect holds a position of primary importance in the development of modern physics. From the time of its first discovery and explanation, it has stimulated the development of quantum mechanics by providing experimental evidence that classical mechanics and electrodynamics were powerless to explain. During the entire early period of quantum theory (from 1920 to 1930), the Compton effect was a central phenomenon against which the theory could be tested. It provided conclusive proof that Albert Einstein’s concept of the photon (introduced early in the 1900’s to explain the photoelectric emission of electrons) as having both energy and momentum was correct. It also indicated that material particles have a wave nature and show interference effects. This wave-particle duality found in both radiation and matter lies at the heart of modern quantum theory.
This remarkable discovery had consequences that went far beyond the photon concept; it became the basis for Werner Heisenberg’s
The Compton effect has played an important role in many diverse areas of science. For example, it has affected radiation shielding in nuclear physics. A beam of radiation is attenuated as it passes through matter as the photons are absorbed or scattered by the material. One of the experimental facts that Compton found was that the relative intensities of the primary and secondary radiation depend on the wavelength of the exciting radiation. The importance of the processes involved in attenuating the beam depends on the photon energies. Information of this type is useful for the design of radiation shielding.
The Compton effect has been used directly in the early diagnosis of osteoporosis,
Scientists have also obtained information about the electronic structure of molecules and crystals from the Compton effect. A Doppler shift
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Boorse, Henry A., and Lloyd Motz. “The Compton Effect.” In The World of the Atom. New York: Basic Books, 1966. Brief description of the Compton effect and its importance to the development of physics. Also features reprints of a 1962 essay on Compton by S. K. Allison originally published in Science and two of Compton’s early papers. -
Crease, Robert P., and Charles C. Mann. The Second Creation: Makers of the Revolution in Twentieth-Century Physics. Rev. ed. New Brunswick, N.J.: Rutgers University Press, 1996. Follows the development of physics from its nineteenth century roots to the enigmatic mysteries of the late twentieth century. Examines characters and personalities as well as the issues of physics. Includes brief discussion of the Compton effect. -
Ford, Kenneth W. The Quantum World: Quantum Physics for Everyone. Cambridge, Mass.: Harvard University Press, 2004. Explains the concepts of quantum physics in nontechnical language for lay readers. Illustrated. -
Heathcote, Niels H. de V. Nobel Prize Winners in Physics, 1901-1950. New York: Henry Schuman, 1953. The chapter on Compton contains a clear, nonmathematical description of the Compton effect. Quotes extensively from Compton’s Nobel lecture. Suitable for readers with little background in physics. -
Hendry, John. The Creation of Quantum Mechanics and the Bohr-Pauli Dialogue. Boston: D. Reidel, 1984. History of quantum mechanics intended for readers who have had at least an introduction to quantum mechanics at the beginning college level. Includes discussion of the Compton effect and its importance to American physicists. -
Massey, Sir Harrie. The New Age in Physics. 2d ed. New York: Harper, 1967. Provides a clear and interesting account of the development of twentieth century physics. Discusses the wave-particle question in the first two chapters, as well as other topics important to an understanding of the Compton effect. -
Stuewer, Roger H. The Compton Effect: Turning Point in Physics. New York: Science History Publications, 1975. Detailed analysis of the theoretical and experimental work in the field of radiation physics related to the background, discovery, and impact of the Compton effect. Accessible to readers with a general background in college-level physics.
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