( ( when ( ( these electrons ) ( return ( to ( a ( lower ( energy level ) ) ) ) ) ) ) ( , ( ( ( ( visible and ) ultraviolet ) light ) ( ( is emitted ) . ) ) ) )
(ROOT (S (SBAR (WHADVP (WRB when)) (S (NP (DT these) (NNS electrons)) (VP (VB return) (PP (TO to) (NP (DT a) (JJR lower) (NN energy) (NN level)))))) (, ,) (NP (ADJP (JJ visible) (CC and) (JJ ultraviolet)) (NN light)) (VP (VBZ is) (VP (VBN emitted))) (. .)))
when these electrons return to a lower energy level, visible and ultraviolet light is emitted.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NNS electrons)) (VP (VB return) (PP (TO to) (NP (DT a) (JJR lower) (NN energy) (NN level)))))) (, ,) (NP (PRP they)) (VP (VBP emit) (NP (NP (DT the) (JJ excess) (NN energy)) (PP (IN in) (NP (NP (DT the) (NN form)) (PP (IN of) (NP (NN light))))))) (. .)))