Question

it is an imaginary part of a spherical mirror mirror it is denoted by capital r

Answer 1

Rays of light parallel to the principal axis of aconcave mirror will appear to converge on a point in front of the mirror somewhere between the mirror's pole and its center of curvature. That makes this a convergingmirror and the point where the rays converge is called the focal point or focus.
The point , on the principal axis, which is equidistant from all points on the reflecting surface of the mirror is called the centre of curvature. The distance along the principal axis from point to point is called the radius ofcurvature of the mirror, and is denoted .