Explore the foundations of quantum mechanics in this lecture from Yale University's Fundamentals of Physics II course. Delve into the double slit experiment and its implications for Newtonian mechanics. Examine the de Broglie relation, connecting wavelength and momentum for photons and electrons. Review experimental evidence supporting Einstein's photon theory of light, including the photoelectric effect and Compton scattering. Discover the wave function and its probability interpretation. Investigate the uncertainty principle and its connection to wave diffraction. Cover key topics such as Young's double slit experiment, the particulate nature of light, particle-wave duality of matter, and the uncertainty principle in this comprehensive 1-hour 14-minute lecture.