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principal axis

Optics. a line passing through the center of the surface of a lens or spherical mirror and through the centers of curvature of all segments of the lens or mirror.
Physics. one of three mutually perpendicular axes of a body about which the moment of inertia is maximum or minimum.
Origin of principal axis
1875-80 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2016.
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British Dictionary definitions for principal axis

principal axis

the line passing through the optical centre and centres of curvature of the faces of a lens or a curved mirror
any of three mutually perpendicular axes about which the moment of inertia of a body is maximum
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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