( Light ( ( can ( ( ( be bent ) ( -LRB- ( refracted -RRB- ) ) ) ( when ( it ( is ( ( ( slowed down ) or ) ( speeded up ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Light)) (VP (MD can) (VP (VB be) (ADJP (JJ bent)) (PRN (-LRB- -LRB-) (VP (VBN refracted)) (-RRB- -RRB-)) (SBAR (WHADVP (WRB when)) (S (NP (PRP it)) (VP (VBZ is) (VP (VP (VBN slowed) (PRT (RP down))) (CC or) (VP (VBN speeded) (PRT (RP up))))))))) (. .)))
Light can be bent (refracted) when it is slowed down or speeded up.
(ROOT (S (NP (VBG Slowing)) (ADVP (RB down)) (VP (VBZ causes) (ADJP (JJ light) (S (VP (TO to) (VP (VB refract)))))) (. .)))