Term for the distance from the center of a lens to the pt. where parallel rays that pass through it converge
The distance between the principal focus and the centre of the 
lens is called ... lens the refracted rays diverge so that they appear to come from 
one point... [DOC] [TLE] Lenses [PAR] Both converge parallel rays to a focal point, have positive focal lengths, and form 
... starting from the tip of the object and passing through the center of the lens.