Correcting Chromatic Aberrations - In a single converging lens, due to chromatic dispersion, different colors are not focused to the same point (see figure to the right). - An achromatic lens or achromat is designed to reduce the effects of chromatic aberration. - A common type of achromat is a doublet consisting of a converging and a diverging lens with different refractive indices and amounts of chromatic dispersion. Commonly, the converging lens (eg. crown glass) has a relatively low refractive index and a relatively low dispersion. The diverging lens (eg. flint glass) has a higher refractive index and dispersion. - The lenses are often cemented together and shaped so that the chromatic aberration of the red and blue light are counterbalanced. Hence, the red and blue light are brought to the same focus (which is slightly different for the middle wavelength, i.e. green light). Hence, a sharper image is produced (see figure below).

Photograph taken with (top) and without (bottom) achromatic lens: