principal planen.
/ˌprɪnsᵻpl ˈpleɪn/, U.S. /ˈˌprɪnsəp(ə)l ˈpleɪn/ 1. A chief plane of reference in a body or system; esp. each of three such planes (often mutually perpendicular), such as ones involving symmetry, forces, etc.
ΚΠ
1715 B. Taylor Linear Perspective ii. 21 Having found the Vanishing Line of some principal Plane, and the Representation of a Remarkable Line in the Design.
1854 Proc. Royal Soc. 7 316 Referring the motion of a system consisting of two planets to axes so chosen that the plane of xy shall always coincide with the principal plane of the system.
1930 H. Shapley Flights from Chaos ix. 74 The orbital plane of the Earth..is near the principal plane of the whole solar system.
1966 McGraw-Hill Encycl. Sci. & Technol. (rev. ed.) X. 150/1 These fringes represent lines making known angles with the principal planes of stress.
1992 S. P. Maran Astron. & Astrophysics Encycl. 21/1 At about 18 kpc from the center, the stellar disk seems to bend out of its principal plane, a phenomenon known as a ‘warping’.
2. Optics. Either of two planes perpendicular to the optic axis of a lens such that the image of an object placed in either of these planes is the same size as the object.
ΚΠ
1887 R. S. Heath Treat. Geom. Optics iv. 63 These planes are planes of unit magnification... The two planes are called the principal planes.
1962 E. Ackerman Biophysical Sci. ii. 30 The strength of a lens (or its power), L, is defined as the reciprocal of the focal length f measured from the corresponding principal plane.
1982 R. D. Overheim & D. L. Wagner Light & Color vii. 172 When we say..that an image is located at a certain distance from the lens, we mean that it is located that distance from a principal plane of the lens.
This entry has been updated (OED Third Edition, June 2007; most recently modified version published online March 2022).
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