( ( Whenever ( ( a wave ) ( ( changes speed ) ( in ( ( passing ( from ( one medium ) ) ) ( to another ) ) ) ) ) ) ( , ( ( part ( of ( that wave ) ) ) ( ( is reflected ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB Whenever)) (S (NP (DT a) (NN wave)) (VP (VBZ changes) (NP (NN speed)) (PP (IN in) (S (VP (VBG passing) (PP (IN from) (NP (CD one) (NN medium))) (PP (TO to) (NP (DT another))))))))) (, ,) (NP (NP (NN part)) (PP (IN of) (NP (DT that) (NN wave)))) (VP (VBZ is) (VP (VBN reflected))) (. .)))
Whenever a wave changes speed in passing from one medium to another, part of that wave is reflected.
(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))) (. .)))