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WordNet
optical aberration

n. an optical phenomenon resulting from the failure of a lens or mirror to produce a good image [syn: aberration, distortion]

Wikipedia
Optical aberration

An optical aberration is a departure of the performance of an optical system from the predictions of paraxial optics. In an imaging system, it occurs when light from one point of an object does not converge into (or does not diverge from) a single point after transmission through the system. Aberrations occur because the simple paraxial theory is not a completely accurate model of the effect of an optical system on light (due to the wave nature of light), rather than due to flaws in the optical elements.

Aberration leads to blurring of the image produced by an image-forming optical system. Makers of optical instruments need to correct optical systems to compensate for aberration.

The articles on reflection, refraction and caustics discuss the general features of reflected and refracted rays.