Geometrical construction relating the effective group index n_g-eff to the effective index n_eff of the fundamental mode of a fiber, with the dependence in normalized spatial frequency λ_c/λ. A double-slope line is drawn from where the tangent to the effective index curve crosses the ordinate axis, and the group index n_g(λ_c/λ₀) equates DS₀(λ_c/λ₀), as shown for λ_c/λ₀ = 0.8, i.e., for λ₀ = 1.25 λ_c. As in Figure 7, one easily sees that n_g-eff is about equal to the refractive index of the core n₁, in the practical domain of use of a single-mode fiber, i.e., λ_c < λ < 1.5 λ_c, or 0.67 < λ_c/λ < 1.