This chapter covers the following topics: Photoelectric Effect and Einstein's Photon Theory; Hydrogen Atom and deBroglieWave. Chapter Contents: 13.1 Introduction 13.2 Photoelectric Effect and Einstein's Photon Theory 13.3 Hydrogen Atom 13.4 de Broglie Wave 13.5 Problems Inspec keywords: wave mechanics; photoelectricity; hydrogen; particle optics Other keywords: photoelectric effect; de BroglieWave; Hydrogen Atom; Einstein's Photon Theory Subjects: Atom and neutron optics; Textbooks; Particle beams and particle optics