Spherical aberration, distortion, coma, and field curvature are purely a function of the shape of the lens surfaces and
an be observed with monochromatic light. There are, however, other aberrations that arise when these optics are used to
transform light containing multiple wavelengths.
The index of refraction of a material is a function of wavelength. Known as dispersion, this is discussed in the section
on Material Properties. From Snell's law it can be seen that light rays of different wavelengths or colors will be
refracted at different angles since the index is not a constant. The figure below shows the result when polychromatic
collimated light is incident on a positive lens element. Because the index of refraction is higher for shorter
wavelengths, these are focused closer to the lens than the longer wavelengths. Longitudinal chromatic aberration is
defined as the axial distance from the nearest to the farthest focal point.
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Lognitudinal chromatic aberration
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As in the case of spherical aberration, positive and negative elements have opposite signs of chromatic aberration.
Once again, by combining elements of nearly opposite aberration to form a doublet, chromatic aberration can be partially
corrected. It is necessary to use two glasses with different dispersion characteristics, so that the weaker negative
element can balance the aberration of the stronger, positive element.
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