in: the paraxial approximation
refers: for a lens to the object focus on the optical axis, from which rays are refracted by the lens to emerge parallel to the optical axis; also to the image focus on the optical axis, to which parallel rays converge after refraction by the lens. [P6.3]
is: for a concave mirror the point (called the focus) on the optical axis, to which rays parallel to the optical axis converge after reflection at the mirror. [P6.3]
is: for a convex mirror the point (called the focus) on the optical axis, from which rays parallel to the optical axis appear to diverge after reflection at the mirror. [P6.3]
See first focal point, second focal point, focal length.
Copyright 1997, The Open University