Essential Interpretation of Invariant Light String Speed and Refraction Mechanism in Media Based on the First Principle Unified Theory of Light Strings

Traditional optical phenomenology holds that the speed of light decreases in transparent media such as water and glass, and regards this conclusion as a basic optical law. However, this view has inherent logical paradoxes: if the propagation speed of light can be continuously reduced by media, it will theoretically lead to the absurd result that light can be completely static.Based on the First Principle Unified Theory of Light Strings, this paper proposes a new essential explanation: the intrinsic propagation speed of free light strings is always constant and equal to the cosmic benchmark speed of light, and will never decelerate in any medium. The apparent slowdown of light speed in media is not the real reduction of light string speed, but the macroscopic equivalent effect caused by the tortuous and detoured propagation path of light strings under the gradient difference of background light string field density.Dense real particles such as protons and electrons will squeeze and repel the background light string field, resulting in lower internal field density of the medium and forming a stable density gradient potential difference at the interface. When light strings obliquely enter the medium, the wavefront deflects continuously and smoothly due to the field density difference, which macroscopically presents the refraction phenomenon. This mechanism is completely homologous with gravitational lens light deflection, realizing the unified interpretation of optical and gravitational phenomena.This paper strictly distinguishes the intrinsic propagation speed of light strings and the macroscopic effective drift speed of optical signals, reasonably compatible with Cherenkov radiation principle, and deduces that Snell's law is only a macroscopic phenomenological approximation of the light string refraction law under weak field conditions. Two testable theoretical predictions are proposed, which provide a new underlying theoretical framework for explaining medium optics phenomena.