Define pole of a mirror
Concave and Convex mirror are generally called spherical mirrors because they are part of hollow sphere as shown in figure. Concave mirror has reflecting surface at the interior surface of sphere. Convex mirror has reflecting surface at the exterior of spherical surface. Principal axis is the imaginary central line at the center of the sphere forming the diameter of the sphere as shown in figure. Pole is the geometrical center of convex or concave mirror where the principal axis meets the mirror surface as shown in figure.