The best replacement for refractive index is wavelength.

The best alternatives to refractive index are rated as following:

★★★★★	wavelength
★★★	brix
★	mole fraction
★	speed of light
	dielectric constant
	index of refraction

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Speaking about refractive index, one probably shouldn't forget about wavelength.

Other search terms related to refractive index and their ratings are:

★★★★★	wavelength
	critical angle

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refractive index

wavelength

brix