on the optical axis, from which rays are refracted by the lens to emerge parallel to the axis. [P6.3]
in: the paraxial approximation
is: for a convex lens, the point on the optical axis, from which rays are refracted by the lens to emerge parallel to the axis. [P6.3]
is: for a concave lens, the point on the optical axis, from which rays which have been refracted parallel to the axis by the lens, appear to emanate. [P6.3]
is also called: first focus or object focus.
See focal length.
Copyright 1997, The Open University