I wrote these articles to complement my articles on quantum mechanics listed in the section “Linear Algebra and Quantum Mechanics,” because I worry that those articles may be too mathematical and allow readers to lose the physical picture. However, those articles can be read more or less independently as well without knowing these articles much. The first two articles do not assume any knowledge of physics. “Bohr model” assumes knowledge of high school physics, such as centripetal force and Coulomb force. “De Broglie’s matter waves” should help to understand my article, “A short introduction to quantum mechanics X: comparison with de Broglie’s matter waves and time-dependent Schr?dinger equation.” “Schr?dinger equation,” “Tunneling,” and “Infinite potential well” assume knowledge of differential equations.