This work presents a structural reexamination of Young’s double-slit experiment by identifying a missing stage in its standard formulation and restoring it within a finite, geometry-based framework. The conventional treatment assumes continuous propagation, neutral boundaries, and observation on a single fixed screen, reducing the experiment to a geometric division of a continuous entity. Using exact geometry, interference is described by the condition , which defines a family of hyperbolas in space. From the triangle inequality, the number of interference fringes for monochromatic light is shown to be finite and exactly determined. The commonly observed linear fringe pattern arises from the far-field approximation and represents only a partial cross-section of this global structure. A central result of this work is the identification of the in-slit stage as a necessary part of the experiment. The slit is treated as a finite region in which particles interact with boundary-defined electromagnetic environments, leading to directional redistribution prior to propagation. This interpretation is supported by an asymmetric single-slit experiment using different materials, which produces a reversible asymmetric pattern inconsistent with a purely geometric model. The framework is extended to limiting and classical cases, including tiny apertures and diffraction phenomena such as the Poisson spot, where symmetry and lack of comparison may conceal boundary-dependent effects. The suppression of interference in which-way detection experiments is interpreted as a modification of the electromagnetic environment within the slit. These results show that interference in Young’s experiment is a finite, spatial, and timing phenomenon that cannot be fully described by post-slit geometry alone. Restoring the in-slit interaction provides a complete factual description and introduces material-dependent effects absent from standard treatments.
dong ZHANG (Fri,) studied this question.