Chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light (lens dispersion), meaning that different colors focus at different distances from the lens. Longitudinal and lateral chromatic aberration of a lens can be seen as "fringes" of color around the image.
Chromatic aberration of a single lens causes different wavelengths of light to have differing focal lengths.
An achromatic doublet has the same focal length for red and blue light.
As can be seen above, chromatic aberration can be minimized by using an Achromatic Lens or Achromat, in which special optical materials with differing dispersion are assembled together to form a compound lens.