<P> While the refractive index affects wavelength, it depends on photon frequency, color and energy so the resulting difference in the bending angle causes white light to split into its constituent colors . This is called dispersion . It can be observed in prisms and rainbows, and chromatic aberration in lenses . Light propagation in absorbing materials can be described using a complex - valued refractive index . The imaginary part then handles the attenuation, while the real part accounts for refraction . </P> <P> The concept of refractive index is widely used within the full electromagnetic spectrum, from X-rays to radio waves . It can also be used with wave phenomena such as sound . In this case the speed of sound is used instead of that of light and a reference medium other than vacuum must be chosen . </P> <P> The refractive index n of an optical medium is defined as the ratio of the speed of light in vacuum, c = 7008299792458000000 ♠ 299 792 458 m / s, and the phase velocity v of light in the medium, </P> <Dl> <Dd> n = c v . (\ displaystyle n = (\ frac (c) (v)).) </Dd> </Dl>

The refractive index of a medium is determined by the ratio of