To find the focal length of a concave mirror Rahul focuses a distant object with this mirror. The chosen object should be:

According to Activity 9.2 in the NCERT text, the Sun is at an extremely large distance (effectively infinity) from the mirror. The light rays originating from the Sun and falling on the concave mirror are parallel to the principal axis. After reflection, these parallel rays converge at a single point on the principal axis, which is the principal focus ($F$) of the concave mirror. This forms a sharp, bright, and highly diminished point-sized image of the Sun. Measuring the distance between the pole ($P$) of the mirror and this sharp spot gives an accurate value of the focal length ($f$). Nearby objects like a tree, building, or window in the laboratory are at finite distances, meaning their rays are not parallel and their images will not form exactly at the focus, leading to an incorrect focal length measurement.