Refractive Interface as a Metric Boundary: Electromagnetic Parameter Transition and Metric Reference Displacement

This paper takes the refractive interface as its starting point and reframes optical refraction as a process of metric-baseline migration at the interface. In classical geometric optics, refraction is described by Snell's law, whose physical basis is the variation of light speed in different media. Without introducing any new physical assumptions, this paper argues within the framework of Maxwell's electromagnetic parameters (ε, μ) that the essence of a dielectric interface is a metric boundary — the variation of electromagnetic parameters causes a shift in the metric baseline across the interface, and refraction is the observable consequence of this metric-baseline migration.The central proposition of this paper is that the geometric properties of a refractive interface are not determined by an externally imposed metric, but are intrinsically determined by the electromagnetic parameters themselves. This framework traces the everyday phenomenon of a straw appearing bent in water back to the metric level rather than stopping at the level of light-speed variation, providing a foundational interface-node model for subsequent studies of metric networks.