F7 by CDGM

$$n(\lambda) = \sqrt{2.60275 - 0.00846789\,\lambda^2 + 0.0246596\,\lambda^{-2} + 0.000744513\,\lambda^{-4} - 4.85328 \times 10^{-6}\,\lambda^{-6} + 4.2592 \times 10^{-6}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[1.17 \times 10^{-6} + 2(2.08 \times 10^{-8})\,\Delta T + 3(-1.11 \times 10^{-10})\,\Delta T^2 + \frac{1.16 \times 10^{-6} + 2(2.78 \times 10^{-10})\,\Delta T}{\lambda - 0.217}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$