( ( As ( light ( ( passes ( fromone medium ) ) ( to another ) ) ) ) ( , ( ( its speed ) ( changes . ) ) ) )
(ROOT (S (SBAR (IN As) (S (NP (NN light)) (VP (VBZ passes) (NP (JJ fromone) (NN medium)) (PP (TO to) (NP (DT another)))))) (, ,) (NP (PRP$ its) (NN speed)) (VP (VBZ changes)) (. .)))
As light passes fromone medium to another, its speed changes.
(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))) (. .)))