The refractive index of materials varies with the wavelength (and frequency) of light. This is called dispersion and causes prisms and rainbows to divide white light into its constituent spectral colors. As the refractive index varies with wavelength, so will the refraction angle as light goes from one material to another. Dispersion also causes the focal length of lenses to be wavelength dependent. This is a type of chromatic aberration, which often needs to be corrected for in imaging systems. In regions of the spectrum where the material does not absorb light, the refractive index tends to "de"crease with increasing wavelength, and thus "in"crease with frequency. This is called "normal dispersion", in contrast to "anomalous dispersion", where the refractive index "in"creases with wavelength. For visible light normal dispersion means that the refractive index is higher for blue light than for red.