Measure the outgoing spectrum at an angle
Compton compared incident X-rays with radiation scattered by graphite. The spectrum of the scattered beam contained a component at a longer wavelength. Moving the observation direction changed the displacement. The measured quantity was a wavelength distribution, not a photograph of a photon striking an electron.
This makes the angle essential evidence. A generic statement that some energy was lost is much weaker than a relation predicting how much wavelength change should accompany a specified scattering direction. Source lines, spectrometer calibration and the separation of spectral components belong to that comparison.
Read the evidence: A. H. Compton (1923), A Quantum Theory of the Scattering of X-rays by Light Elements, Phys. Rev. 21, 483
Keep both conservation laws
Consider one incident photon and a free electron initially at rest. Conserve total energy and the vector momentum, using relativistic energy for the recoiling electron. The resulting shift is shown below. The angle is between the incoming and outgoing photon directions; the wavelength difference means outgoing minus incoming.
Planck’s constant divided by the electron mass and the vacuum speed of light sets the scale. It is about 2.43 picometres. Forward scattering gives no shift, a right angle gives that scale, and backscattering gives twice it. These are limits of the ideal kinematics, not new measurements.
Read the evidence: A. H. Compton (1923), A Quantum Theory of the Scattering of X-rays by Light Elements, Phys. Rev. 21, 483NIST, 2022 CODATA recommended constants — Compton wavelength
Why a wave-only comparison was under pressure
In the elementary classical Thomson picture, a stationary free electron driven by weak monochromatic radiation reradiates at the incident frequency. The displaced component therefore asks for more than this elastic response. Compton’s model ties the lower outgoing photon energy to the recoil electron while also satisfying momentum conservation.
The conclusion is specific to the competing description. Classical electromagnetic fields can carry momentum, and observing radiation pressure alone does not establish Compton kinematics. Here the discriminating evidence is the wavelength–angle relation together with the collision energy accounting, not the mere existence of a force.
Read the evidence: A. H. Compton (1923), A Quantum Theory of the Scattering of X-rays by Light Elements, Phys. Rev. 21, 483
Not every electron in graphite is free
The derivation treats the initial electron as unbound and at rest. Compton explicitly limited his argument by neglecting binding forces. In a real solid, initial electron momentum, binding, coherent elastic scattering and instrumental resolution affect the spectrum. A component near the original wavelength does not erase the displaced one.
Nor does the shift formula predict all intensities or multiple scattering. A reader should keep the peak displacement, the line shape and the differential scattering rate as separate tests. Improving one part of a model does not automatically validate the others.
Read the evidence: A. H. Compton (1923), A Quantum Theory of the Scattering of X-rays by Light Elements, Phys. Rev. 21, 483
Connect the evidence without merging the instruments
Photoemission asks how incoming light ejects an electron from a material, so its energy balance includes a work function. Compton scattering follows an outgoing photon as well as an electron. A work function is not a replacement for the electron recoil energy in the free-electron derivation.
The existing photoemission Lab offers a comparison of energy bookkeeping. It is not a Compton spectrometer, does not acquire angle-resolved X-ray spectra and cannot supply the historical graphite record. The 1923 paper and the 1927 award belong to different dates; neither date should be inferred from the simulator.
Read the evidence: A. H. Compton (1923), A Quantum Theory of the Scattering of X-rays by Light Elements, Phys. Rev. 21, 483Nobel Prize in Physics 1927 — official award record