This quantity equals the square root of one plus the electric susceptibility by the Ewald–Oseen extinction theorem. Cauchy expressed this quantity (20[1])as a Laurent series (20[1])with only even (20[1])non-positive terms in an equation that inspired a formula for the square of this quantity, the Sellmeier equation. Values (20[1])of this quantity at levels (20[1])labeled C, d, and F are used to calculate a substance’s (*) Abbe (10[1])number. (10[1])This quantity (10[2])is (0[1])anisotropic (10[1])and polarization-sensitive in birefringent (10[2])materials. (10[3])The product of this quantity (10[1])in a medium and the sine (10[1])of the (10[1])angle of incidence (10[2])is (10[1])constant (10[2]0[1])across a boundary (10[1])per Snell’s law. (10[1]0[2])For 10 points, (10[2])name this dimensionless optical quantity, the ratio of the speed of light (10[1])in two media. (10[1]0[1])■END■ (10[4])