( ( ( ( ( ( Within ( such ( a medium ) ) ) ( , ( ( the speed ) ( is constant ) ) ) ) , ) but ) ( light ( ( changes ( ( its speed ) ( when ( it ( ( ( moves ( from ( one medium ) ) ) ( -LRB- ( ( say air ) -RRB- ) ) ) ( to another ) ) ) ) ) ) ( -LRB- ( ( say glass ) -RRB- ) ) ) ) ) . )
(ROOT (S (S (PP (IN Within) (NP (PDT such) (DT a) (NN medium))) (, ,) (NP (DT the) (NN speed)) (VP (VBZ is) (ADJP (JJ constant)))) (, ,) (CC but) (S (NP (NN light)) (VP (VBZ changes) (NP (NP (PRP$ its) (NN speed)) (SBAR (WHADVP (WRB when)) (S (NP (PRP it)) (VP (VBZ moves) (PP (IN from) (NP (CD one) (NN medium))) (PRN (-LRB- -LRB-) (S (VP (VB say) (NP (NN air)))) (-RRB- -RRB-)) (PP (TO to) (NP (DT another))))))) (PRN (-LRB- -LRB-) (S (VP (VB say) (NP (NN glass)))) (-RRB- -RRB-)))) (. .)))
Within such a medium, the speed is constant, but light changes its speed when it moves from one medium (say air) to another (say glass).
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NN light)) (VP (VBZ passes) (PP (IN from) (NP (CD one) (NN medium))) (PP (TO to) (NP (DT another)))))) (, ,) (NP (PRP it)) (VP (VBZ changes) (NP (NN speed))) (. .)))