1. (in optics) A defect in the image formed by a lens or curved mirror. In chromatic aberration the image formed by a lens (but not a mirror) has coloured fringes as a result of the different extent to which light of different colours is refracted by glass. It is corrected by using an achromatic lens. In spherical aberration, the rays from the object come to a focus in slightly different positions as a result of the curvature of the lens or mirror. For a mirror receiving light strictly parallel with its axis, this can be corrected by using a parabolic surface rather than a spherical surface. Spherical aberration in lenses is minimized by making both surfaces contribute equally to the ray deviations, and can (though with reduced image brightness) be reduced by the use of diaphragms to let light pass only through the centre part of the lens. See also astigmatism; coma.

Aberration.

2. (in astronomy) The apparent displacement in the position of a star as a result of the earth’s motion round the sun. Light appears to come from a point that is slightly displaced in the direction of the earth’s motion. The angular displacement α‎=v/c, where v is the earth’s orbital velocity and c is the speed of light.