what is is geometric centre of a mirror why it is called as?

Dear student 

I will talk in sense of geometric centre of spherical mirror.

The geometric centre of a spherical mirror is called its pole. The centre of the hollow sphere for which the mirror is a part, is called the centre of curvature. The line joining the centre of curvature and the pole is the principal axis. A light ray incident on a spherical mirror, after reflection appears to pass through the principal focus in the case of a convex mirror and passes through the focus in the case of concave mirror. The diameter of the spherical mirror gives the measure of its aperture.

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