When a light wave is reflected at the interface between two semi-infinite media, the relative phase between the reflected and incident amplitudes is a fundamental optical effect. Another fundamental phase relation arises in a symmetric beam splitter between its reflection and transmission amplitudes. Both effects originate from the phase structure of wave scattering, although their underlying connection is not always clearly understood. As a pedagogical investigation, this paper systematically analyzes these phase relations using simple one-dimensional potential models in quantum mechanics and provides intuitive physical explanations. For a symmetric square barrier, the reflected and transmitted amplitudes necessarily differ in phase by ±π/2, with the reflection phase referenced at the interface on the incident side and the transmission phase referenced at the interface on the exit side. The result is further extended to general symmetric barriers, for which the detailed form of the barrier determines only whether the phase difference is +π/2 or -π/2. The role of this phase structure in the second-order interference of two identical particles is then examined. Through this sequence of analysis, half-wave loss at a reflecting interface, the phase relation in a symmetric beam splitter, and multiparticle quantum interference are incorporated into a unified wave-scattering framework. This treatment provides a useful pedagogical reference for related topics in quantum mechanics and optics courses.