( ( Light waves ) ( ( are ( refracted ( when ( ( crossing ( ( ( the boundary ) ( from one ) ) ( transparent ( medium ( into another ) ) ) ) ) ( because ( ( ( the speed ) ( of light ) ) ( is ( different ( in ( different media ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Light) (NNS waves)) (VP (VBP are) (VP (VBN refracted) (SBAR (WHADVP (WRB when)) (S (VP (VBG crossing) (S (NP (NP (DT the) (NN boundary)) (PP (IN from) (NP (CD one)))) (ADJP (JJ transparent) (NN medium) (PP (IN into) (NP (DT another))))) (SBAR (IN because) (S (NP (NP (DT the) (NN speed)) (PP (IN of) (NP (NN light)))) (VP (VBZ is) (ADJP (JJ different) (PP (IN in) (NP (JJ different) (NNS media)))))))))))) (. .)))
Light waves are refracted when crossing the boundary from one transparent medium into another because the speed of light is different in different media.
(ROOT (S (NP (JJ Different) (NNS media)) (VP (VBP affect) (NP (NP (DT the) (NN speed)) (PP (IN of) (NP (NN light))))) (. .)))