( ( ( The piece ) ( of glass ) ) ( ( can ( be ( ( altered ( in shape ) ) ( to ( ( become ( ( either ( ( ( a convex ) or ) ( a ( concave lens ) ) ) ) ( showing ( how ( ( the beams ) ( ( converge and ) diverge ) ) ) ) ) ) accordingly ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN piece)) (PP (IN of) (NP (NN glass)))) (VP (MD can) (VP (VB be) (VP (VBN altered) (PP (IN in) (NP (NN shape))) (S (VP (TO to) (VP (VB become) (S (NP (CC either) (NP (DT a) (NN convex)) (CC or) (NP (DT a) (JJ concave) (NN lens))) (VP (VBG showing) (SBAR (WHADVP (WRB how)) (S (NP (DT the) (NN beams)) (VP (VB converge) (CC and) (VB diverge)))))) (ADVP (RB accordingly)))))))) (. .)))
The piece              of glass can be altered in shape to become either a convex or a concave              lens showing how the beams converge              and diverge accordingly.
(ROOT (S (NP (NP (DT The) (NN lens)) (PP (IN of) (NP (DT the) (NN eye)))) (VP (VBZ is) (S (NP (DT a) (-LRB- -LRB-) (NN n) (-RRB- -RRB-) (NN convex)) (VP (VB shape)))) (. .)))