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Compton effect
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X-ray wavelength increases after scattering by electrons.
Δλ = (h/mc)(1−cosθ)X-rays scattering off electrons emerge with longer wavelength: Δλ = (h/mₑc)(1 − cosθ). It can only be explained by photon–electron collisions that conserve energy and momentum.
At θ = 90° the shift is 2.43 pm.
Visible light shifts are too small to notice.
In more detail
In the Compton effect, X-rays scatter from electrons and lose energy, increasing their wavelength. The shift depends on the scattering angle, and the result confirmed photon momentum.
Real-world uses
- Compton scattering is used in medical imaging and radiation physics.
- Gamma-ray astronomy detectors use Compton scattering.
- Physicists studying X-rays use the Compton shift to measure the energy and momentum transferred to electrons.
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Frequently asked questions
Why does the wavelength increase?
The photon transfers some energy and momentum to the electron, lowering its frequency.
Is the shift always the same?
No, it depends on the scattering angle, reaching a maximum at 180 degrees.
Why does the wavelength increase after scattering?
The photon gives some momentum and energy to the electron, so its frequency falls and its wavelength grows.
Related topics in Quantum Mechanics
Threshold frequency · Stopping potential · Photon momentum · Pair production · de Broglie hypothesis · Davisson–Germer experiment