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Refractivity

In atmospheric applications, refractivity is defined as N = n – 1, often rescaled as either N = 10 (n – 1) or N = 10 (n – 1); the multiplication factors are used because the refractive index for air, n deviates from unity by at most a few parts per ten thousand.

Molar refractivity, on the other hand, is a measure of the total polarizability of a mole of a substance and can be calculated from the refractive index as A = M ρ ⋅ n 2 − 1 n 2 + 2 , {\displaystyle A={\frac {M}{\rho }}\cdot {\frac {n^{2}-1}{n^{2}+2}}\,} where ρ is the density, and M is the molar mass.