Chromatic Aberration

1. (optics) An optical aberration in which an image has coloured fringes, caused by differential refraction of light of different wavelengths.

In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point.^[1] It is caused by dispersion: the refractive index of the lens elements varies with the wavelength of light. The refractive index of most transparent materials decreases with increasing wavelength.^[2] Since the focal length of a lens depends on the refractive index, this variation in refractive index affects focusing.^[3] Chromatic aberration manifests itself as "fringes" of color along boundaries that separate dark and bright parts of the image.

Astrophysics:

Is an aberration that occurs in lenses but not in mirrors. This happens because a material’s index of refraction depends on the wavelength. The focal length of a lens is therefore different at different wavelengths. The images at different wavelengths are formed in different places.

The focal length is different for different wavelengths

It can be corrected, somewhat with a two-lens system (An Achromat).