( ( The lens ) ( ( may ( be ( ( convex or ) concave ) ) ) . ) )
(ROOT (S (NP (DT The) (NN lens)) (VP (MD may) (VP (VB be) (ADJP (JJ convex) (CC or) (JJ concave)))) (. .)))
The lens may be convex or concave.
(ROOT (S (NP (DT A) (JJ concave) (NN lens)) (VP (VBZ is) (NP (NP (JJR thicker)) (PP (IN at) (NP (DT the) (NNS edges)))) (SBAR (IN than) (S (NP (PRP it)) (VP (VBZ is) (PP (IN in) (NP (DT the) (NN middle))))))) (. .)))
A concave lens is thicker at the edges than it is in the middle.