Lecture
This lecture introduces the concept of photons, particles with zero mass, within the framework of relativistic dynamics. It explores how the study of electromagnetic radiation led to the idea of photons, illustrated through the analysis of a collision between an electron and a photon. The history of photon discovery, from Planck's work on electromagnetic radiation to Einstein's interpretation as a gas of particles, is discussed. The lecture also covers the Compton effect, demonstrating the collision between X-rays and electrons, and the conservation of energy and momentum in such interactions.