(a) A number of rays parallel to the principal axis after reflection from a concave mirror meet at a point on the principal axis or appear to come from a point after reflection from a convex mirror on the principal axis. This is called principal focus.
(b) When the object is placed beyond C.
(c) f = - 12 cm, u = - 6 cm, v = ?
Using the expression
1/f = 1/v - 1/u , we have
1/-12 = 1/v - 1/-6
v = 12 cm