Noun
a lens such that a parallel beam of light passing through it is caused to diverge or spread out
Source: WordNetA convex plus a concave lens (f 1 > 0 > f 2 ) produces a positive magnification and the image is upright. Source: Internet
The appearance is similar to a single-arch dam but with a distinct vertical curvature to it as well lending it the vague appearance of a concave lens as viewed from downstream. Source: Internet