Principal section

(Crystallog.)

a plane passing through the optical axis of a crystal.

principal section

[′prin·sə·pəl ′sek·shən] (mathematics) A normal section at a given point on a surface whose curvature has a maximum or minimum value. (optics) A plane in a crystal that contains the crystal's optic axis and the ray of light under consideration. Also known as principal plane.

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