( ( When ( light ( enters ( a gem ) ) ) ) ( , ( it ( ( ( ( changes course ) and ) ( is ( ( slowed down ) ( -LRB- ( refracted -RRB- ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NN light)) (VP (VBZ enters) (NP (DT a) (NN gem))))) (, ,) (NP (PRP it)) (VP (VP (VBZ changes) (NP (NN course))) (CC and) (VP (VBZ is) (VP (VBN slowed) (ADVP (RB down)) (PRN (-LRB- -LRB-) (VP (VBN refracted)) (-RRB- -RRB-))))) (. .)))
When light enters a gem, it changes course and is slowed down (refracted).
(ROOT (S (NP (VBG Slowing)) (ADVP (RB down)) (VP (VBZ causes) (ADJP (JJ light) (S (VP (TO to) (VP (VB refract)))))) (. .)))