Chromatic aberration (also called chromatic distortion and spherochromatism) is a defect in a lens in which the various colors of the spectrum are not brought to the same focus. It is caused by dispersion because the refractive index of the lens elements varies with the wavelength of light. In particular, the refractive index of most transparent materials decreases with increasing wavelength.
The result is fringing – the formation of a colored halo along boundaries that separate dark and bright parts of the image. This problem seriously affected the performance of refracting telescopes for centuries and was the reason that so many refractors were built with large focal ratios: longer focal length lenses show a less chromatic error.
A better solution came with the introduction of corrective elements, using at least two different types of glass, into a compound lens. An achromatic lens corrects for red and blue light, whereas an apochromatic lens corrects for at least red, blue, and green. Reflecting telescopes are free from this type of aberration.