Comptessa

Meet Comptessa, an X-ray photon who bounces off an electron and comes away a little redder.

In 1923 Arthur Compton fired X-rays at a target and found that scattered X-rays had longer wavelengths than the incoming ones. Classical waves could not explain this. Treat the X-ray as a particle carrying momentum h/λ, though, and it becomes a billiard-ball collision: the photon gives some energy and momentum to an electron, and its wavelength grows by an amount that depends only on the scattering angle θ.

The scale is the Compton wavelength of the electron, h/(m_e c) ≈ 2.426 picometres. A photon scattered at 90° gains exactly that much wavelength; one bounced straight back gains twice that. For visible light, with wavelengths around 500 nanometres, the shift is far too small to notice, which is why X-rays were needed.

The result was strong evidence that light carries momentum in quanta. Compton shared the 1927 Nobel Prize in Physics for it.

The formula assumes a free electron at rest. Electrons bound in atoms add a spread to the measured shifts.