★ The point at which rays parallel to principal axis coming from infinity appear to converge after being refracted from concave lens iscalledtheprincipal focus ofconcave lens.
★ Principal focus-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. Thisiscalledprincipalfocus.