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The radius of curvature of a spherical mirror is actually the radius of the circle which the spherical mirror is a part. It can also be defined as the distance between the centre of the curvature and the pole of the mirror which is on the principal axis.

Now, using the concept which says that the radius of the curvature of the mirror will not be altered when the spherical mirror is cut horizontally that is along the optical axis. Also, the use of the relation between the radius of curvature and the focal length of the mirror to be describe the location of the image formed by the lower half of the mirror. The image will be formed by the lower half of the mirror.

Now, the radius of curvature of the spherical mirror is twice the focal length of the mirror. The focal length of the lower part of the mirror will be same as initial case because the radius of curvature will bot be changed when it cut horizontally. Hence by mirror equation location of the image formed by the lower part of the mirror is also as earlier case.