This lecture introduces the principles of quantum mechanics, focusing on the quantization of energy and the photoelectric effect. It covers the concept of photons, energy quantification, and the behavior of electrons in metals. The instructor discusses the wave-particle dualism and the emission spectrum of atoms, emphasizing the discrete properties of light and the transition between energy states. Various experiments at the origin of quantum mechanics are presented, highlighting the importance of energy thresholds and the characteristics of different electromagnetic radiations.