Chapter-9|Ray Optics and Optical Instrument|NCERT 12th Physics:
We consider and object AB placed on the principal axis beyond the centre of curvature C of a concave mirror of small aperture. A ray AM from the object travels parallel to the principal axis and after reflection from the mirror it passes through focus F. Another ray AP is incident on the pole P of the mirror and is reflected a long PA´ in accordance with the law of reflection so that ∠APB=∠B´PA´. The two reflected rays meet at point A´. Thus A´ is the real image of A. The image of any point on AB will lie on a corresponding point of A´B´.
Hence, A´B´ is the real image of AB formed by reflection from the concave mirror.