principal-axis

principal axis

noun
1.
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.
2.
Physics. one of three mutually perpendicular axes of a body about which the moment of inertia is maximum or minimum.

Origin:
1875–80

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
principal axis
 
n
1.  the line passing through the optical centre and centres of curvature of the faces of a lens or a curved mirror
2.  any of three mutually perpendicular axes about which the moment of inertia of a body is maximum

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