Light incident upon a molecule induces vibrating dipoles, and the greater the refractive index at a particular wavelength, the greater is the dipolar induction. The interaction of light photons with the polarizable electrons of a dielectric causes a reduction in the velocity of light. The dielectric constant, being a measure of polarizability, is greatest when dipolar interactions with light are proportionally large. The refractive index for light of long wavelengths, n∞ , is related to the dielectric constant for a nonpolar molecule, , by the expression