When a photon collides with a free electron at rest and the direction of motion of the photon changes,
A) the magnitude of the momentum of the photon does not change.
B) the momentum of the electron does not change.
C) the kinetic energy of the electron does not change.
D) the total energy of the photon does not change.
E) both the magnitude of the momentum and the total energy of the photon decrease.

Question

Quizplus · Accepted Answer

When a photon collides with a free electron (Compton scattering), the photon transfers some of its energy and momentum to the electron. As a result, the photon's energy (and thus its frequency) decreases, which means both the magnitude of its momentum and its total energy decrease. This is because the energy of a photon is directly related to its frequency (E = hf), and its momentum is related to its wavelength ($p = \frac{h}{\lambda}$). A change in frequency (due to the change in energy) implies a change in wavelength, and thus a change in momentum.

When a Photon Collides with a Free Electron at Rest