The Compton Effect can be defined as the shift in the wavelength of a photon when colliding with a free electron. The phenomenon was discovered by Arthur Compton in 1923. His discoveries suggest that photons resemble particles of matter. This provided further evidence to the quantum theory and particle nature of light. In 1927, Compton won the Nobel Prize in Physics for his experiments. Many questions were raised about the nature of photons after Einstein’s discoveries in 1905. Compton, curious to whether or not the law of conservation of momentum applied to photons, decided to study collisions between photons and electrons. He sought free electrons as these would be ideal to conduct his studies with since there would be no outside forces acting upon the electrons. However, free electrons rarely exist in nature. Alternatively, Compton chose a metal with a very low work function and a high frequency photon source. This way the energy required to eject the photoelectron would be considered insignificant compared to the total energy of the system. Compton used X – rays for his experiment given that they produce very high frequency photons.
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« The Photoelectric EffectHow an Amplifier Works » Through his experiments Compton discovered that when a high energy photon collides with an electron there is a transfer of energy. Also, he discovered that the electron and the lower energy photon scatter after the collision. The equation that describes the conservation of energy in the collision is hf= hf’+ ½ mev2 (Figure 2) where hf is the energy of the scattered photon and ½ mev2 is the kinetic energy of the electron The photon loses energy in the collision because it is transferred to the electron in the form of kinetic energy. Since the energy of a photon is dependent upon its frequency, this transfer of energy suggests that the frequency has decreased, and therefore a subsequent increase in the wavelength. The observed wavelength shift cannot be explained by the classical wave theory of light. Compton wanted to determine whether momentum was conserved in the collision. However, photons have no mass, so in order to do this he had to go to Einstein’s formula E=mc2. Compton used m= E/c2 in place of mass for the momentum equation p=mv. Through the use of these two equations, along with the universal wave equation, he was able to come up with the equation for the momentum of a photon; p=h/λ (Figure 3) . Just like a collision between two particles of matter, Compton was able to use this equation to prove that momentum was conserved in the collision between the photon and electron. The Compton Effect has many applications in modern science. The most notable area of use is in the field of radio-biology. This is because Compton scattering is the most likely reaction between x-rays and atoms of the body during radiation. It is also used in material physics and in the study of gamma spectroscopy. The Compton Effect was a very important discovery in physics. It led scientists to accept the quantum theory of light. Thompson scattering, which supports the classical wave theory of light, cannot explain the shift in wave length which occurs in these collisions. The only way that the Compton Effect can be explained is through the idea that light behaves as particles.

Figure 1: Momentum Equation for Photon
P=hλ
Figure 2: Conservation of Energy in Photon-Electron Collision hf= hf’+ ½ mev2
Figure 3: Deriving Photon Momentum Equation
E=mc2 m= E/c2
P=mv P=( E/c2)(V) V=c
P=E/c E=hf
P=hf/c fλ=V=c
P=hf/fλ
P=h/λ