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Chapter 5:
Secondary Spectrum and Apochromats
When longitudinal chromatic aberration has been corrected, we normally find that the focal positions for three wavelengths still do not coincide. This effect is known as secondary spectrum and is due to the nonlinearity of the curve of refractive index as a function of wavelength. More precisely, it stems from a difference between the nonlinearity of the positive (crown) element and the negative (flint) element of an achromat.
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