In the 1923 Compton X-ray scattering experiment, the wavelength of X-rays changed after they collided with electrons.
Arthur Holly Compton directed X-rays at graphite and measured the radiation scattered at different angles. The scattered X-rays had longer wavelengths than the incoming radiation, and the amount of the shift depended on the scattering angle.
Compton explained the result by treating the X-ray beam as photons carrying energy and momentum. In a collision, a photon transfers some energy and momentum to an electron, so the outgoing photon has lower energy and therefore a longer wavelength.
Classical wave theory could not account for the angle-dependent wavelength shift. The observation became important evidence for the particle-like properties of electromagnetic radiation and earned Compton the 1927 Nobel Prize in Physics.