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Physics LibreTexts

4: The Photon

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  • 4.1: Light as a Stream of Particles
  • 4.2: Compton Scattering
    Compton scattering refers to the scattering of light off of free electrons. Experimentally, it’s impossible to create a target of completely free electrons. However, if the incident photons have energy much greater than the typical binding energies of electrons to atoms, the electrons will be “knocked off” of the atoms by the photons and act as free particles. Therefore, Compton scattering typically refers to scattering of high energy photons off of atomic targets.
  • 4.3: Pair Production
    In pair production, a photon can simply vanish and in its place a matter-antimatter pair of particles can appear. This phenomenon is a wonderful illustration of the fact that mass is not conserved, since the mass of the electron and positron can be created from the energy of the massless photon. Of course, the photon must have sufficient energy to create the rest masses of the two new particles.
  • 4.4: Photons and Matter
    The photons can be absorbed by the electrons, scatter from the electrons, and create pairs of particles by interacting with the nucleus. Which of these processes will occur for any particular photon and how far will the photon penetrate the matter before one of these processes occurs. These questions require an understanding of the probability of each of these processes occurring. The probability of a particular process occurring is represented by the cross section for that process.
  • 4.5: Activities
  • 4.6: The Laser Elevator (Project)
    In a 2002 paper, Laser Elevator: Momentum Transfer Using an Optical Resonator, Thomas R. Meyer et. al. talk about a neat way to get a lot more speed out of light reflection than with a regular solar sail. The basic physics are pretty simple, and it’s a fun subject to think about. When a photon hits a solar sail, it gives the sail momentum.

Contributors and Attributions


This page titled 4: The Photon is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Paul D'Alessandris.

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