( This ( ( is ( ( the reason ) ( why ( ( a ( concave lens ) ) ( is ( called ( a ( diverging lens ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (VBZ is) (NP (NP (DT the) (NN reason)) (SBAR (WHADVP (WRB why)) (S (NP (DT a) (JJ concave) (NN lens)) (VP (VBZ is) (VP (VBN called) (NP (DT a) (VBG diverging) (NN lens)))))))) (. .)))
This is the reason why a concave lens is called a diverging lens.
(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.