The Compton effect

The Compton effect
The Compton effect

The last century has witnessed a series of scientific discoveries and revolutions that have radically transformed our understanding of the universe. Among these epochal changes, the Compton effect emerges as one of the key revelations that have redefined the nature of light and matter. At a time when the frontiers of quantum physics were rapidly evolving, this discovery offered a new lens through which to observe and interpret the interaction between radiation and matter.

But what exactly is the Compton effect? In essential terms, it is a shift in the wavelength of a photon, or particle of light, when it collides with a free electron and is deflected from its original trajectory. This phenomenon not only confirmed the corpuscular nature of light but also provided a clear understanding of the conservation of energy and momentum in electromagnetic interactions.

The Compton experiment



Compton’s experiment is called the zero experiment because it is unequivocally interpretable; photons really exist and interact with electrons individually .

Compton’s experimental apparatus consisted of:

  • an X-ray tube which, passing through a monochromator and a collimator, made the beam monochromatic and parallel with�=7,09⋅10−11 �
  • a graphite target
  • an X-ray detector .

The monochromatic, parallel beam hit the graphite target and scattered at different angles. The scattered radiation was captured by the detector, and the values were ​​entered into a graph.

From experimental observations, it was noted that, for α = 90°, a small portion of radiation had an average wavelength greater than the radiation incident on the graphite block. As can also be seen from the graph alongside,�’=7,31⋅10−11 �, this means that there is a wavelength variation of0,22⋅10−110,22⋅1 to the incident radiation. From the experimental evidence, a minimal variation in the amplitude of the scattered wave was expected, but not in its intensity! Other experimental tests demonstrated that it depended neither on the target material nor on the wavelength of the radiation used in the experiment. What does it depend on then? Where does the energy dispersed in the form of wavelength go?

Interpretation of the Compton effect

Compton interpreted the diffuse wavelength variation, to Einstein’s theory , as an interaction of photons with matter, justifying the increase in wavelength as a consequence of the loss of part of the momentum of the incident wave.

Relativistic momentum, or incident radiation, depends on the frequency, according to the formula: the=ℎ ��. The single photon, hitting an electron and freeing it from the graphite, loses momentum. The scattered electromagnetic wave that has interfered and is collected by the detector will have a final momentum��less than�the, as in elastic collisions. Since the momentum of an electromagnetic wave depends only on the frequency, the scattering frequency ‘f’ will be less than the accident frequency, and consequently, the wavelength’λof the scattered radius will be greater than that of the incident ray.

Compton demonstrated the photon-electron interaction via Einstein’s special relativity, unequivocally proving the existence of photons and their individual interaction with matter via elastic collisions.

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