Gravitational Lensing as Anisotropic Refraction in an Effective Hyper-Elastic Continuum

Classical refractive models of gravity historically failed to reproduce the full generalrelativistic prediction for gravitational light deflection, yielding only half the observedvalue. We revisit this problem using modern continuum mechanics, modeling the vacuum as an effective hyper-elastic continuum rather than a scalar fluid. We show thatgravitational compression induces not only a density gradient but also an anisotropic reduction of shear stiffness through the photo-elastic (Poisson) response. When both effects are included, the resulting refractive-index gradient reproduces the full Schwarzschild light-deflection angle in the weak-field, static limit. This result demonstrates that gravitational lensing is not uniquely diagnostic of spacetime curvature, but is observationally degenerate with anisotropic optical propagation in a mechanically responding medium.