extraordinary partial dispersi

The human eye can sense monochromatic light wavelengths within the range of 400nm (purple) to 700nm (red). Within this range, the difference in index of refraction between two different wavelengths is called partial dispersion. Most ordinary optical materials have similar partial dispersion characteristics. However, partial dispersion characteristics differ for some glass materials, such as glass that exhibits larger partial dispersion at short wavelengths, FK glass which features a small index of refraction and low dispersion characteristics, fluorite, and glass that exhibits larger partial dispersion at long wavelengths. These types of glass are classified as having extraordinary partial dispersion characteristics. Glass with this property is used in apochromatic lenses to compensate chromatic aberration.