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(ROOT (S (PP (IN For) (NP (DT a) (NN plane) (NN wave) (NN perturbation))) (, ,) (NP (NP (DT the) (NN velocity) (NN field)) (PP (IN with) (NP (NN direction)))) (VP (VBZ reversing) (PP (IN in) (NP (NP (NNS crests)) (VP (ADVP (CC and)) (VBG troughs) (-LRB- -LRB-) (NP (NP (VB see)) (VP (VBN Fig))))))) (. .)))
For a plane wave perturbation, the velocity field with direction reversing in crests and troughs (see Fig.
(ROOT (S (PP (IN In) (NP (DT an) (JJ electromagnetic) (NN wave))) (, ,) (NP (DT the) (NNS crests) (CC and) (NNS troughs)) (VP (VBP represent) (S (VP (VBG oscillating) (NP (NNS fields))))) (. .)))