( ( That is ) ( , ( ( it ( 's ( ( a wave ) ( of ( oscillating ( electromagnetic fields ) ) ) ) ) ) . ) ) )
(ROOT (S (INTJ (DT That) (VBZ is)) (, ,) (S (NP (PRP it)) (VP (VBZ 's) (NP (NP (DT a) (NN wave)) (PP (IN of) (S (VP (VBG oscillating) (NP (JJ electromagnetic) (NNS fields)))))))) (. .)))
That is, it's a wave of oscillating electromagnetic fields.
(ROOT (S (SBAR (IN Because) (S (NP (NP (DT the) (NNS fields)) (SBAR (WHNP (WDT that)) (S (VP (VBP make) (PRT (RP up)) (NP (DT an) (JJ electromagnetic) (NN wave)))))) (VP (VBP are) (PP (IN at) (NP (NP (JJ right) (NNS angles)) (PP (PP (TO to) (NP (DT each) (JJ other))) (CC and) (PP (TO to) (NP (DT the) (NN direction)))))) (SBAR (IN that) (S (NP (DT the) (NN wave)) (VP (VBZ travels))))))) (, ,) (NP (DT an) (JJ electromagnetic) (NN wave)) (VP (VBZ is) (VP (VBN considered) (S (NP (DT a) (-LRB- -LRB-) (NN n) (-RRB- -RRB-) (NN transverse) (NN wave))))) (. .)))