Chromatic aberration is a defect in a lens in which the various colors
of the spectrum are not brought to the
same focus. Blue light, for example, is refracted (see refraction)
more than red light when it passes through a lens and hence comes to a focus
inside that of red light. The result is fringing – the formation of a colored halo around 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 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.
|Chromatic aberration produces coloured fringes around the lens edges, and parts of the image may not be sharp [C]. This aberration occurs with single lenses because they behave like prisms and bend blue light more than red light [A]. Combining the lens with a weaker concave lens [B] made of a different glass cancels out this dispersion effect, and both red and blue rays are brought to the same focus to produce a sharper, more distinct image.
AND OPTICAL PHENOMENA