Chromatic aberration is caused by the dispersion of the lens material, the variation of its refractive index n with the wavelength of light.
Since the focal length f of a lens is dependent on n, if follows that different wavelengths of light will be focused to different positions. Chromatic aberration of a lens is seen as fringes of color around the image. It can be minimised by using an achromatic doublet or achromat in which two materials with differing dispersion are bonded together to form a single lens.
This reduces the amount of chromatic aberration over a certain range of wavelengths, though it does not produce perfect correction. By combining more than two lenses of different glasses, the correction can be stil better as in apochromatic lens or apochromat.
The use of achromats was an important step in the developement of the optical microscope.