( ( As ( light ( ( passes ( from ( ( one ( medium ( -LRB- ( ( such ( as air ) ) -RRB- ) ) ) ) ( into ( another medium ) ) ) ) ) ( -LRB- ( ( such ( as glass ) ) -RRB- ) ) ) ) ) ( , ( it ( ( ( 's ( speed changes ) ) ( according ( to ( ( the index ) ( of ( refraction ( of ( the ( two media ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN As) (S (NP (NN light)) (VP (VBZ passes) (PP (IN from) (NP (NP (CD one) (NN medium) (PRN (-LRB- -LRB-) (PP (JJ such) (IN as) (NP (NN air))) (-RRB- -RRB-))) (PP (IN into) (NP (DT another) (NN medium))))) (PRN (-LRB- -LRB-) (PP (JJ such) (IN as) (NP (NN glass))) (-RRB- -RRB-))))) (, ,) (NP (PRP it)) (VP (VBZ 's) (NP (NN speed) (NNS changes)) (PP (VBG according) (PP (TO to) (NP (NP (DT the) (NN index)) (PP (IN of) (NP (NP (NN refraction)) (PP (IN of) (NP (DT the) (CD two) (NNS media))))))))) (. .)))
As light passes from one medium (such as air) into another medium (such as glass), it's speed changes according to the index of refraction of the two media.
(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))) (. .)))