How do I derive the expression for the group index $N_g$ as below
$$N_g=n-\lambda\frac{dn}{d\lambda}$$
using the Cauchy dispersion relation below? $$n=n_{-2}(hf)^{-2}+n_0+n_2(hf)^2+n_4(hf)^4$$
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2Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. – Community Jun 01 '22 at 19:47
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1You don't need Cauchy. – J.G. Jun 02 '22 at 07:05