Another way in which light can help reactions go is by changing the symmetry of a molecule. This can be tested with the Woodward–Hoffmann rules. Some pericyclic reactions go with heat, some go only with light.
ʌ n ʌ ð ə ɹ WORD_BOUNDARY w eɪ WORD_BOUNDARY ɪ n w ɪ t̠ʃ WORD_BOUNDARY l aɪ t WORD_BOUNDARY k æ n WORD_BOUNDARY h ɛ l p WORD_BOUNDARY ɹ ɪ æ k ʃ ə n z WORD_BOUNDARY ɡ oʊ WORD_BOUNDARY ɪ z WORD_BOUNDARY b aɪ WORD_BOUNDARY t̠ʃ eɪ n d̠ʒ ɪ ŋ WORD_BOUNDARY ð ə WORD_BOUNDARY s ɪ m ə t ɹ i WORD_BOUNDARY ə v ə WORD_BOUNDARY m ɑ l ɪ k j uː l WORD_BOUNDARY ð ɪ s WORD_BOUNDARY k æ n WORD_BOUNDARY b iː WORD_BOUNDARY t ɛ s t ɪ d WORD_BOUNDARY w ɪ ð ð ə WORD_BOUNDARY w ʊ d w ə ɹ d WORD_BOUNDARY h ɑ f m ə n WORD_BOUNDARY ɹ uː l z WORD_BOUNDARY s ʌ m WORD_BOUNDARY p ɛ ɹ ɪ s aɪ k l ɪ k WORD_BOUNDARY ɹ ɪ æ k ʃ ə n z WORD_BOUNDARY ɡ oʊ WORD_BOUNDARY w ɪ ð WORD_BOUNDARY h iː t WORD_BOUNDARY s ʌ m WORD_BOUNDARY ɡ oʊ WORD_BOUNDARY oʊ n l i WORD_BOUNDARY w ɪ ð WORD_BOUNDARY l aɪ t WORD_BOUNDARY