( ( In ( a ( concave lens ) ) ) ( ( the light ) ( ( has ( to ( ( travel ( further ( ( through ( the middle ) ) then ) ) ) ( through ( the sides ) ) ) ) ) . ) ) )
(ROOT (S (PP (IN In) (NP (DT a) (JJ concave) (NN lens))) (NP (DT the) (NN light)) (VP (VBZ has) (S (VP (TO to) (VP (VB travel) (ADVP (ADVP (RBR further)) (PP (IN through) (NP (DT the) (NN middle)) (ADVP (RB then)))) (PP (IN through) (NP (DT the) (NNS sides))))))) (. .)))
In a concave lens the light has to travel further through  the middle then through the sides.
(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.