ENOxford University Press Oxford University Press A Dictionary of Physics © Oxford University Press 2000, 2003 Published by Oxford University Press, Great Clarendon Street, Oxford OX2 6DP Oxford is a registered trade mark of Oxford University Press All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, or under terms agreed with the appropriate reprographics rights organisation. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department Oxford University Press. Look up a word or term Look up an abbreviation Look up a person Look up an affix A ab- A prefix attached to the name of a practical electrical unit to provide a name for a unit in the electromagnetic system of units (see electromagnetic units), e.g. abampere, abcoulomb, abvolt. The prefix is an abbreviation of the word ‘absolute’ as this system is also known as the absolute system . Compare stat-. In modern practice both absolute and electrostatic units have been replaced by SI units. Back - New Search Abelian group See group theory. Back - New Search aberration 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 = v/c, where v is the earth's orbital velocity and c is the speed of light. Top Back - New Search abscissa See Cartesian coordinates. Back - New Search absolute 1 Not dependent on or relative to anything else, e.g. absolute zero. 2 Denoting a temperature measured on an absolute scale , a scale of temperature based on absolute zero. The usual absolute scale now is that of thermodynamic temperature; its unit, the kelvin, was formerly called the degree absolute (°A) and is the same size as the degree Celsius. In British engineering practice an absolute scale with Fahrenheit-size degrees has been used: this is the Rankine scale. Top Back - New Search absolute expansivity See expansivity. Back - New Search