Deviation of light rays by lenses or curved mirrors which causes the images to be blurred. Spherical aberration occurs because curvature in a lens or mirror causes rays falling on the outer edges to be brought to a focus at a different point than those falling on the middle. This makes the images formed appear blurred. Chromatic aberration, which occurs in lenses but not mirrors, is the failure of a lens to focus all colours (wavelengths) of light in the same plane; the image appears blurred and shows rainbow-coloured fringes around the edges. See also astigmatism.

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