Chapter 38: Photons: Light Waves Behaving as Particles

The photoelectric effect demonstrates that light energy is quantized, with photons carrying energy E = hf, and that electrons are only ejected when photon energy exceeds the material's work function, regardless of light intensity. X-ray production through bremsstrahlung radiation occurs when high-energy electrons decelerate rapidly upon striking metal targets, producing photons with maximum energy equal to the kinetic energy of the incident electrons. Compton scattering provides compelling evidence for photon momentum through the collision of X-ray photons with free electrons, resulting in wavelength shifts that follow precise mathematical relationships and demonstrate conservation of energy and momentum in photon-electron interactions. Pair production and annihilation processes illustrate the interconversion between matter and energy, where high-energy gamma photons create electron-positron pairs near atomic nuclei, while the reverse process converts particle pairs back into photons. The chapter establishes wave-particle duality as a fundamental principle, showing that light simultaneously exhibits wave properties like interference and diffraction alongside particle behaviors in emission and absorption processes. The Heisenberg uncertainty principle emerges as a consequence of quantum mechanics, imposing fundamental limits on the simultaneous measurement of complementary properties like position and momentum, or energy and time, thereby replacing deterministic classical predictions with probabilistic quantum descriptions of physical phenomena.