Summary
This chapter marks the turning point from classical to quantum physics, demonstrating that many quantities once considered continuous are quantized. Experiments with blackbody radiation, the photoelectric effect, and Compton scattering revealed that electromagnetic energy is emitted and absorbed in discrete packets called photons. The Bohr model subsequently provided a framework for understanding atomic energy levels and spectral lines, while phenomena such as pair production and annihilation underscored the deep connection between energy and matter. Overall, these concepts form the foundation of modern quantum physics.