After a brief review of photon interactions to prepare for the problem set, the ways in which ions (charged particles) interact with nuclei are introduced. The formula for stopping power (energy loss per distance traveled through matter) is derived from a physical model, and compared with the full stopping power formula. The two are strikingly similar. Integrating the inverse of stopping power also gives the range of ions in matter—unlike photons, ions stop at nearly fixed distances in matter.