An Etymological Dictionary of Astronomy and Astrophysics
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A measure of the ability of a medium to separate different colors of light. It is defined by: (n₂ - n₁)/(n - 1), where  n₁ and n₂ are refractive indices at two specified widely differing wavelengths, and n is the → index of refraction for the average of these wavelengths.

→ dispersive; → power.