( ( ( The speed ) ( of light ) ) ( ( changes ( as ( it ( passes ( through ( ( a medium ) ( such ( as air ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN speed)) (PP (IN of) (NP (NN light)))) (VP (VBZ changes) (SBAR (IN as) (S (NP (PRP it)) (VP (VBZ passes) (PP (IN through) (NP (NP (DT a) (NN medium)) (PP (JJ such) (IN as) (NP (NN air))))))))) (. .)))
The speed of light changes as it passes through a medium such as air.
(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))) (. .)))