constringence, in optics, a measure of the dispersive power of a transparent substance for the visible spectrum. Letting nF, nD, and nC represent the indices of refraction for light of the wavelengths λF (blue), λD (yellow), and λC (red), the constringence (commonly denoted by the Greek letter nu, ν) is defined by the equation ν = (nD - 1)/(nF - nC) and represents, in a prism, the ratio of the mean deviation of the spectrum as a whole from the direction of the incident light beam to the angular fanning out of the spectrum itself.