The most direct evidence of the corpuscular properties of light was obtained in 1923 by Arthur H. Compton, observing X-ray scattering. Compton used the Bragg x-ray spectrometer for spectral analysis of scattered rays from various angles after focusing on target. It found a component of the same wavelength λ as the incident radiation and another with wavelength λ' > λ that incident λ, where λ' varies with the scattering angle. The displacement between the incident and scattered wavelengths is called the Compton shift: Δλ = λ' - λ. He tested the Einstein hypothesis by treating the x-ray in terms of photons, that is, as particles of energy. In addition to energy, electromagnetic radiation carries momentum.
Compton admitted that X-rays can be scattered by the electrons of the atom, so binding energy would be negligible, meaning each electron would behave as a free particle. In this way, the scattering would be a collision between a photon of energy E and moment p and a free electron, initially at rest. Due to the "retreat" of the electron in the collision, the photon would have E' > E, so the wavelength λ' > λ, as observed. With this interpretation of Compton we have the equation below that relates the Compton shift with the scattering angle.
Δλ = λ' - λ = [h/(m0c)](1 - cosθ)
the constant (h/m0c), where m0 is the electron resting mass, h is the Planck constant and c the speed of light, is called the electron Compton wavelength and is calculated from known and established constants. Thus, the indication of the corpuscular nature of light was established through the Compton Effect by its quantized treatment by the use of the Einstein relation and the treatment of this "retreat" of the electron as a collision of particles. As already mentioned, scattered radiation also contains a wavelength component λ. Its appearance can be explained as resulting from the scattering not by an electron, but the atom as a whole. Since the mass of the target's carbon atom is much larger than the mass of the electron, the corresponding displacement is negligible.
Compton effect, increase in wavelength of X-rays and other energetic electromagnetic radiations that have been elastically scattered by electrons; it is a principal way in which radiant energy is absorbed in matter. The effect has proved to be one of the cornerstones of quantum mechanics, which accounts for both wave and particle properties of radiation as well as of matter.
NUSSENZVEIG, H. Moysés. Curso de Física Básica: Ótica, Relatividade, Física Quântica. vol. 4, 1ª ed. Editora Bluncher, São Paulo, 1998. p. 254-257.
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