( ( ( ( ( ( ( The speed ) ( of light ) ) ( is ( constant ( in ( a vacuum ) ) ) ) ) , ) but ) ( it ( changes ( when ( ( the light ) ( travels ( through ( another medium ) ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NP (DT The) (NN speed)) (PP (IN of) (NP (NN light)))) (VP (VBZ is) (ADJP (JJ constant) (PP (IN in) (NP (DT a) (NN vacuum)))))) (, ,) (CC but) (S (NP (PRP it)) (VP (VBZ changes) (SBAR (WHADVP (WRB when)) (S (NP (DT the) (NN light)) (VP (VBZ travels) (PP (IN through) (NP (DT another) (NN medium)))))))) (. .)))
The speed of light is constant in a vacuum, but it changes when the light travels through another medium.
(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))) (. .)))