The Compton effect (sometimes called Compton scattering) occurs when an x ray collides with an electron. In 1923, Arthur H. Compton did experiments bouncing x rays off the electrons in graphite atoms. Compton found the x rays that scattered off the electrons had a lower frequency (and longer wavelength) than they had before striking the electrons. The amount the frequency changes depends on the scattering angle, the angle that the x ray is deflected from its original path.
Imagine playing pool. Only the cue ball and 8 ball are left on the table. When the cue ball strikes the 8 ball, which was initially at rest, the cue ball is scattered at some angle. It also loses some of its momentum and kinetic energy to the 8 ball as the 8 ball begins to move. The x-ray photon scattering off an electron behaves similarly. The x ray loses energy and momentum to the electron as the electron begins to move. The energy and frequency of light and other electromagnetic radiation are related so that a lower frequency x-ray photon has a lower energy. The frequency of the x ray decreases as it loses energy to the electron.
In 1905, Albert Einstein explained the photoelectric effect, the effect that causes solar cells to produce electricity, by assuming light can occur in discrete particles, photons. This photon model for light still needed further experimental confirmation. Compton's x ray scattering experiments provided additional confirmation that light can exhibit particle-like behavior. Compton received the 1927 Nobel Prize in physics for his work. Additional experiments show that light can also exhibit wave-like behavior and has a wave particle duality.