When the object is placed at the focus of the lens, the refracted rays will be incident normally on the silvered surface. So, they will retrace their path.
Hence, the image will be formed at the location of the object. In this way, the combination behaves as a concave mirror of radius of curvature $(R)=20\, cm$.
$\therefore f=\frac{R}{2}=10\, cm$
A spherical mirror is a mirror which has been cut out of a spherical surface.
There are two kinds of spherical mirrors:
Convex Mirror
Concave Mirror
Spherical Mirrors
Concave Mirror
Concave mirrors are also called converging mirrors, because in these types of mirrors, light rays converge at a point after impact and reflect back from the reflective surface of the mirror.
Convex Mirror
The convex mirror has a reflective surface that is curved outward. Regardless of the distance between the subject and the mirrors, these mirrors are "always" virtual, upright and reduced.
