( ( A ( convex lens ) ) ( ( is ( ( a lens ) ( that ( is ( thicker ( in ( ( the middle ) ( than ( at ( its edges ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN convex) (NN lens)) (VP (VBZ is) (NP (NP (DT a) (NN lens)) (SBAR (WHNP (WDT that)) (S (VP (VBZ is) (NP (NP (JJR thicker)) (PP (IN in) (NP (NP (DT the) (NN middle)) (PP (IN than) (PP (IN at) (NP (PRP$ its) (NNS edges)))))))))))) (. .)))
A convex lens is a lens that is thicker in the middle than at its edges.
(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.