Further support for the photon model of electromagnetic waves comes from Compton scattering, in which x rays scatter from electrons, changing direction and frequency in the process. Classical electromagnetic wave theory cannot explain the change in frequency of the x rays on scattering, but the photon model can. Suppose an x-ray photon is moving to the right. It has a collision with a slow-moving electron, as shown. The photon transfers energy and momentum to the electron, which recoils at a high speed. The x-ray photon loses energy, and the photon energy formula E = hf tells us that its frequency must decrease. The collision looks very much like the collision between two particles.
When the x-ray photon scatters from the electron,
A. Its speed increases.
B. Its speed decreases.
C. Its speed stays the same.
