Velocity of light in a medium is inversely proportional to its refractive index. Likewise wavelength is directly proportional to refractive index. Impressed electrostatic potentials across a refractive medium change its refractive index (DC Kerr).... and likewise directly modify the parameters just mentioned, within that medium.
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Refractive index does not necessarily refer to visible light, for example silicon carbide is not transparent in the visible, but is quite transparent in the THz region, 0.1 to 10 THz (3 mm to 30 micron lambda). See Naftaly et al: