What is the Compton Effect?

When a photon, usually from the X-Ray spectrum, collides with an electron, or any other charged particle, the wavelength of the scattered X-Ray increases due to the principles of conservation of momentum and energy. When the photon comes into contact with the electron, some of its initial energy and momentum are transferred to the charged particle.

As such, the scattered photon has less energy than the incident photon and thus, a lower frequency and a higher wavelength, due to their inversely proportional relationship. This phenomenon is known as the Compton Effect and its implications constitute a solid base for the quantum theory of light, introduced in 1905 by physicist Albert Einstein, which will be further explored in this article.

Compton effect
The math of the Compton effect (Image: HyperPhysics)

The experiment conducted by Arthur Compton

As Huygens had conclusively proved in the course of the 17th century, before the notion of the photon was introduced or even accepted, the nature of light was widely accepted to be that of a wave. However, in 1923, physicist Arthur H. Compton studied light-matter interaction in a way that would confirm a newly established implication of quantum theory: the wave-particle duality of light. He would later, in 1927, receive the Nobel prize in physics for this discovery.

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Arthur Compton

As part of his experiment, Compton was directing X-Rays (electromagnetic waves with short wavelengths, on the high energy part of the spectrum) at atoms. A photon, the smallest unit of electromagnetic radiation, the quantum of light, would then come into contact with a stationary valence electron (an electron located on the outer shell of an atom, at its highest energy level). When studying these collisions, Compton observed that the EM waves had a slightly shorter wavelength before the collision and that depending on the scale of this shift in wavelength, the electron would be knocked out of its orbit, scattered, at a certain angle, ionizing the atom in the process.

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Illustration of the Compton effect

The implications of Compton’s findings

As a firsthand proof of the particular nature of light, the results of Compton’s experiment further confirmed the photoelectric effect, proposed by Einstein in 1905, and which firmly established the validity of the quantum theory of light, stating that light is constituted of individual units which can carry certain quantities of energy depending on the frequency of oscillation of light. The higher the frequency, the higher the energy of the photon.

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The Compton effect showed that light is made up of photons

If the prepossessed definition of light, based solely on wavelike properties, were to be correct, the observations of Compton’s experiment would have yielded different observations. No change in wavelength in the scattered X-Ray should have been observed because no change in frequency would have been recorded and the EM wave should simply have caused the electron to oscillate at the same frequency.

However, basing himself on the principles of conservation of momentum and energy, and the assumption of an elastic collision (one where no energy is lost to the surroundings), if the electron started to oscillate due to energy transferred from the X-Ray, the latter will have lost some, for no energy can be created. This was in line with Compton’s observations: the increase in wavelength of the wave. Such a phenomenon was in line with the predictions made by Einstein’s quantum theory of light in the scenario and, as such, confirmed that light consisted of photons.

Derived concepts from the Compton Effect

Two principles that stem directly from the Compton effect are the Compton shift and Compton wavelength. The Compton shift is the change in wavelength of the X-Ray as it collides with the electron. Whereas the Compton wavelength is the final wavelength the X-Ray attains as a result of the collision and is directly dependent on the mass of the charged particle, the speed of light, as well as Planck’s constant which relates a photon’s energy to its frequency.

The Compton effect remains one of the most important experiments in physics.

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Explained in simple language. It was a smooth read. Thank you


[…] two years later, physicist Arthur H. Compton conducted an experiment that confirmed the validity of Einstein’s theory and this concept of the wave-particle duality of […]

William Bliss
William Bliss

You might find it interesting to know what the locus of possible scatter directions looks like in a 3D (2+1) momentum versus energy diagram.

If z axis is energy, and x & y are momentum, the incoming photon is on the null cone, say above the +x axis, while the electron is on the z axis.

All the possible scattered photons form an elliptical locus which includes the incoming photon. The ellipse is in one plane which has a slight angle upward in the +x direction as it intersects the light cone.

Here’s the cool part. The locus of all the possible scattered electron “momenergies” forms an identical ellipse, which includes the electron’s rest energy mc^2, and this ellipse is on a parallel plane which intersects the electron’s relativistic momentum hyperbola.

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