( ( An ( electromagnetic wave ) ) ( ( is ( ( ( a combination ) ( of ( ( ( an ( oscillating ( electric field ) ) ) and ) ( oscillating ( magnetic field ) ) ) ) ) ( that ( ( ( ( propagates ( through space ) ) , ) away ) ( from ( its source ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT An) (JJ electromagnetic) (NN wave)) (VP (VBZ is) (NP (NP (DT a) (NN combination)) (PP (IN of) (NP (NP (DT an) (VBG oscillating) (JJ electric) (NN field)) (CC and) (NP (VBG oscillating) (JJ magnetic) (NN field)))) (SBAR (WHNP (WDT that)) (S (VP (VBZ propagates) (PP (IN through) (NP (NN space))) (, ,) (ADVP (RB away)) (PP (IN from) (NP (PRP$ its) (NN source)))))))) (. .)))
An electromagnetic wave is a combination of an oscillating electric field and oscillating magnetic field that propagates through space, away from its source.
(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))))) (. .)))