Compton scattering formula

Explore the Compton scattering formula, its physical interpretation, applications, and an example calculation in this informative article.

Introduction to Compton Scattering Formula

Compton scattering is a significant phenomenon in the field of particle physics and quantum mechanics. It occurs when a high-energy photon interacts with a free, stationary electron, resulting in a change in the photon’s energy and direction. The Compton scattering formula, derived by Arthur Holly Compton in 1923, provides a way to mathematically describe this process.

Key Components of the Compton Scattering Formula

The Compton scattering formula is based on the conservation of energy and momentum principles. It relates the initial and final energies of the photon, as well as the scattering angle, to predict the final energy of the scattered photon.

  1. Initial energy of the photon (E0): The energy of the incident photon before the scattering event.
  2. Final energy of the photon (E): The energy of the scattered photon after the interaction with the electron.
  3. Scattering angle (θ): The angle between the initial and final directions of the photon.
  4. Compton wavelength of the electron (λC): A fundamental constant, given by λC = h / (me c), where h is the Planck’s constant, me is the electron’s mass, and c is the speed of light.

Compton Scattering Formula

The Compton scattering formula can be expressed as:

Δλ = λ – λ0 = (h / (me c)) (1 – cos(θ))

where Δλ is the change in wavelength of the photon, λ is the final wavelength of the photon, λ0 is the initial wavelength of the photon, and θ is the scattering angle.

Physical Interpretation

The Compton scattering formula describes how the wavelength of a photon changes as a result of its interaction with a free electron. This change in wavelength, or shift, is known as the Compton shift. A greater shift corresponds to a larger energy transfer between the photon and the electron, which in turn results in a larger scattering angle. The formula also reveals the dependence of the Compton shift on the electron’s mass, emphasizing the particle-like nature of the interaction.

Significance and Applications

The discovery of Compton scattering was a pivotal moment in the development of quantum mechanics. It provided experimental evidence for the dual nature of light, acting as both a particle and a wave. Furthermore, the Compton scattering formula has been employed in various scientific and technological applications, including:

  • Understanding the behavior of X-rays and gamma rays in different media.
  • Medical imaging techniques, such as computed tomography (CT) and positron emission tomography (PET).
  • Astronomy, through the study of cosmic rays and the cosmic microwave background radiation.

Example Calculation of Compton Scattering

Let’s consider an example to illustrate the application of the Compton scattering formula. We will calculate the Compton shift and the final energy of a scattered photon after interacting with a free electron.


  • Initial energy of the photon, E0 = 1 MeV (mega-electronvolt)
  • Scattering angle, θ = 60°

Step 1: Convert the initial energy to wavelength

λ0 = (h * c) / E0

Using h = 4.135667696e-15 eV·s (Planck’s constant), c = 2.99792458e8 m/s (speed of light), and E0 = 1e6 eV, we get:

λ0 ≈ 1.239841984e-12 m

Step 2: Calculate the Compton shift

Δλ = (h / (me c)) (1 – cos(θ))

Using me = 9.10938356e-31 kg (electron mass), h = 6.62607015e-34 J·s (Planck’s constant), c = 2.99792458e8 m/s (speed of light), and θ = 60°, we get:

Δλ ≈ 2.427449247e-12 m

Step 3: Calculate the final wavelength

λ = λ0 + Δλ

λ ≈ 3.667291231e-12 m

Step 4: Calculate the final energy of the photon

E = (h * c) / λ

Using h = 4.135667696e-15 eV·s (Planck’s constant) and c = 2.99792458e8 m/s (speed of light), we get:

E ≈ 0.337793918 MeV


The Compton shift in this example is approximately 2.43e-12 m, and the final energy of the scattered photon is approximately 0.34 MeV.

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