For obtaining an enlarged upright image of an object, the object is placed between focus (F) and pole (P) of the concave mirror.

A ray AD parallel to the principal axis, gets reflected at D and then passes through the focus F. A second ray AE passing through the centre of curvature C strikes the mirror normally at point E and gets reflected back along the same path.

Now, two reflected rays DF and EC are diverging rays and, therefore, do not intersect each other on the left side. The reflected rays DF and EC are produced backwards (as shown by dotted lines). On producing backwards, they appear to intersect at point A’ behind the concave mirror. Thus, A’ is the virtual image of point A of the object. To get the complete image of the object we draw A’B’ perpendicular to the axis. Thus, A’B’ is the image of the object AB formed by the concave mirror. The image is bigger in size than the object, hence it is an enlarged image.