Question

Define Principal axis ^.^^.^

Answer 1

Answer : the line with respect to which a spherical mirror or lens system is symmetrical and which passes through both the center of the surfaces and their centers of curvature

Answer 2

IN PARABOLA:

The line segment through the focus connecting two points on the parabola and perpendicular to the principal axis. AB is the latus rectum. A parabola whose vertex is at the origin and principal axis is either the x-axis or the y-axis

IN LENS AND MIRROR:

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.