A PHOTON SCATTERS. AN ELECTRON RECOILS.
The Compton effect.
Launch a photon at a free electron. Follow the wavelength change and find where the energy goes.
The collision
Incoming photon Scattered photon Electron
The collision occurs at animation time 1. Travel is slowed enormously; distances are schematic. After the collision, photon and electron displacements preserve their physical speed ratio.
Where does the energy go?
Conserved in every collisionScattered photonElectron kinetic energy
The bar splits the incident photon energy only. The electron’s rest energy is present before and after and cancels from the energy balance. Tiny energy fractions are not enlarged in the bar.
Compare the wavelengths
Equal horizontal scales; wave spacing shows the actual wavelength ratio. These are schematic profiles, not the photon’s size or path.
Wavelength shift against angle
For the same scattering angle, the absolute wavelength increase is independent of the incoming frequency.
A longer wavelength. A lower photon energy.
θ is the angle between the incoming and scattered photon directions; h is Planck’s constant, mₑ is the electron mass and c is the speed of light. λ and λ′ are the wavelengths before and after scattering. E and E′ are the photon energies; K is the electron’s kinetic energy.
Maximum shift: backscatter
At 0° there is no wavelength shift or energy transfer. At 180° the wavelength increase reaches twice the electron Compton wavelength, approximately 4.85 pm.
Why X-rays work better
A shift of a few picometres is substantial compared with an X-ray wavelength. It is a tiny fraction of a visible-light wavelength, so a violet photon stays violet rather than turning red.
About this model
A single photon scatters from one free electron initially at rest. Photon energy and momentum, relativistic electron recoil and energy conservation are calculated together. For unpolarised light, the photon scattering direction is sampled from the Klein–Nishina angular distribution, using uniform solid-angle proposals. Energy and momentum conservation then determine the scattered energy and electron recoil. The diagram is shown in the plane of each collision; its orientation around the incoming beam is not displayed. Electron binding, many-particle effects and other interaction processes are excluded.
Only visible photons use approximate visible-spectrum colours. Infrared, ultraviolet, X-ray and gamma photons use symbolic colours and are labelled. Their actual wavelengths and energies appear in the readings. Schematic wave profiles share one scale within each comparison, but different incident-frequency settings use a different scale so the waves remain readable.
Changing frequency samples a new outcome at that frequency and pauses at the current animation time. New collision or replay samples another outcome; pause and time scrubbing preserve the current event. Each launch is conditioned on a collision occurring; the chance of a photon missing the electron is not simulated. Reference: OpenStax: The Compton Effect; OpenMC: Compton scattering and Klein–Nishina sampling.
Explore the Compton effect
Change the incoming photon frequency and launch a photon at a free electron initially at rest. Compare incident and scattered wavelengths, follow electron recoil and inspect the energy transferred in each collision. Pause or scrub through the event to compare the particles.
Each new collision samples a photon scattering direction from the Klein–Nishina distribution for unpolarised light. Energy and momentum conservation determine the recoil and scattered photon energy. X-ray, gamma-ray and violet-light presets show why the relative wavelength shift depends on the incoming photon energy. Colours outside the visible spectrum are symbolic.