( ( ( This change ) ( in speed ) ) ( ( causes ( ( ( the direction ) ( of ( a ( light beam ) ) ) ) ( to ( change ( as ( it ( ( passes ( from ( one material ) ) ) ( to another ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT This) (NN change)) (PP (IN in) (NP (NN speed)))) (VP (VBZ causes) (S (NP (NP (DT the) (NN direction)) (PP (IN of) (NP (DT a) (JJ light) (NN beam)))) (VP (TO to) (VP (VB change) (SBAR (IN as) (S (NP (PRP it)) (VP (VBZ passes) (PP (IN from) (NP (CD one) (NN material))) (PP (TO to) (NP (DT another)))))))))) (. .)))
This change in speed causes the direction of a light beam to change as it passes from one material to another.
(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))) (. .)))