F13 by CDGM

$$n(\lambda) = \sqrt{2.56957 - 0.00760403\,\lambda^2 + 0.0249681\,\lambda^{-2} + 0.000334502\,\lambda^{-4} + 5.68154 \times 10^{-5}\,\lambda^{-6} + 6.07326 \times 10^{-7}\,\lambda^{-8}}$$

$$\frac{dn}{dT} = \frac{n_0^2 - 1}{2\,n_0}\left[-1.89 \times 10^{-6} + 2(2.03 \times 10^{-8})\,\Delta T + 3(-1.74 \times 10^{-10})\,\Delta T^2 + \frac{1.05 \times 10^{-6} + 2(9.12 \times 10^{-10})\,\Delta T}{\lambda - 0.237}\right] \Delta T = T - T_0, \quad T_0 = 20\,°\text{C}$$