achromatic doublet

A pair of lenses made of glasses of with different dispersion properties that are combined to correct for axial chromatic aberration at two wavelengths (in the red and blue).

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Chromatic Aberration

Chromatic aberrations are wavelength-dependent artifacts that occur because the refractive index of every optical glass formulation varies with wavelength. When white light passes through a simple or complex lens system, the component wavelengths are refracted according to their frequency. In most glasses, the refractive index is greater for shorter (blue) wavelengths and changes at a more rapid rate as the wavelength is decreased.