Term for the distance from the center of a lens to the pt. where parallel rays that pass through it converge
.... "special" rays 
to locate the image: the ray (1) passing through the center of the lens will ... (
parallel to the optical axis) is bent so it passes through the focal point on the other 
side... [DOC] [TLE] Chapter 24 notes [PAR] object distance: s, the distance from the object to the mirror.