An
achromatic lens is actually a combination of two lenses. The light enters through a
convex lens, and then passes through a
concave lens. The two lenses are also made of materials with two different
refractive indexes. This is
assembled in such a way as to minimize
chromatic aberration.
The
reason behind this is that a normal lens will not bring the
focus point of light consisting of different wavelengths into exactly the same point. The
focal length depends on
refraction and
index of refraction of the material used to make the lens. Lenses consisting of a single element, and having a single
refractive index, could bring only a specific
wavelength of color to a specific
focus point.
Having a combination of lenses with two distinct
refractive indexes, otherwise known as a
doublet for chromatic aberration, means that light of two specific wavelength can be brought to a given focus point. The amount of
chromatic aberration depends on the
dispersion of the glass.
The use of a strong lens made from a low
dispersion glass like
crown glass coupled with a weaker,
high dispersion glass like
flint glass can correct the chromatic aberration for two colors. Using materials with specific refractive indexes it is possible to bring the
focus points of two
specific wavelengths of light to a single
specified point in space.
Partial credit to m_turner and http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/aber2.html