---
title: "The Compton Effect and Photon Momentum"
description: "X-rays scattered from electrons come back with a longer wavelength that depends only on the scattering angle, which is what a collision between two particles predicts. The photon therefore carries mom"
canonical: https://lightmysky.com/learn/science/the-compton-effect-and-photon-momentum-mt_wt7kdaof_P
source: https://lightmysky.com/learn/science/the-compton-effect-and-photon-momentum-mt_wt7kdaof_P.md
retrieved: 2026-09-12
---

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# The Compton Effect and Photon Momentum

X-rays scattered from electrons come back with a longer wavelength that depends only on the scattering angle, which is what a collision between two particles predicts. The photon therefore carries momentum h over lambda.

Subject: Science · Area: Quantum & Modern Physics · Ages 18 to 19
Page: https://lightmysky.com/learn/science/the-compton-effect-and-photon-momentum-mt_wt7kdaof_P

## Ready when they can

- Applies conservation of energy and momentum to a photon-electron collision
- Predicts the wavelength shift from the scattering angle and shows it does not depend on the incident wavelength
- Explains why the effect is invisible for visible light on a free electron

## Lesson: Light bounces like a particle

In 1923 Arthur Compton aimed X-rays, the same kind doctors use to picture bones, at graphite and measured what bounced off. The scattered rays came back with slightly longer wavelengths, and the shift grew at larger scattering angles. A pure wave scattering off charge should have kept the same wavelength.

The pattern makes sense if each X-ray travels as a packet that collides with one electron the way two balls collide. The photon hands part of its energy and momentum to the electron, so the photon that leaves carries less energy and a longer wavelength. The shift depends only on the scattering angle, never on the starting wavelength. For visible light on a free electron the shift is far too tiny to notice next to the long wavelength, so X-rays were needed to see it.

**Example.** A photon has no mass yet carries momentum equal to Planck constant divided by wavelength. Short wavelength means high momentum and high energy through E equals h times f. Starlight pushing comet dust away from the Sun is the same momentum at work on a grand scale.

**Tip.** The shift depends only on the scattering angle, never on the starting wavelength. For visible light on a free electron the shift is far too tiny to notice next to the long wavelength. That is why the effect demanded X-rays to be seen.

**Recap.** X-rays scattered off electrons shift with angle, proving photons carry momentum.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Blackbody Radiation and the Quantum of Energy](https://lightmysky.com/learn/science/blackbody-radiation-and-the-quantum-of-energy-mt_B2Bk2IAOa9)
- [Elastic and Inelastic Collisions](https://lightmysky.com/learn/science/elastic-and-inelastic-collisions-mt_JMwUuh1YUq)

## Opens up

- [The Wavefunction and the Born Probability Rule](https://lightmysky.com/learn/science/the-wavefunction-and-the-born-probability-rule-mt_elzmbBzmKE)