Refractive (or phase) index n (solid line curve), and group index n_g (dashed line curve) of silica, as a function of the wavelength λ in a vacuum. At 1.55 μm, the slope of the tangent to the curve n(λ), i.e., dn/dλ or n′(λ), equates minus 0.012 μm⁻¹, and gives the chromatic, or first-order, dispersion; the one to the curve n_g(λ), i.e., dn_g/dλ or n′_g(λ), equates plus 0.007 μm⁻¹, and gives the group velocity dispersion, or second-order dispersion. Since 1/c = 1 ns/300 mm, the dispersion parameter D(1.55 μm) = (1/c) · n′_g(1.55 μm) equates + 23 ps/(nm km).