Estimate refractive index n($\lambda$) using Cauchy’s equation from $n_d$ and Abbe number $V_d$ (at the d-line, 587.6 nm). Pick a material preset or enter custom values; the tool solves $A$, $B$, $C$ from the Fraunhofer F/d/C lines, plots n vs. wavelength (200 nm to 1000 nm, extendable to 2000 nm). It reports dispersion ($dn/d\lambda$) with a reference table. Note: Cauchy is most accurate over limited transparent bands (typically visible); for broader ranges or near absorption, prefer the Sellmeier model.