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, it 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, because all colors cannot be focused at a single common position on the optical axis.
However, there exists a point, called circle of least confusion, where this effect can be minimized.
Chromatic aberration can be further 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 still better as in apochromatic lens or apochromat.
Also, special glasses have been developed, notably containing the mineral called fluorspar, which have so low dispersion that using two lenses already give a very good correction.
The use of achromats was an important step in the development of the optical microscope.
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