Question: which chemical makes it possible for a photograph to be exposed to be open to white light
[{'answer': 'silver halide', 'provenance': [{'wikipedia_id': '204408', 'title': 'Photographic processing', 'start_paragraph_id': 9, 'start_character': 93, 'end_paragraph_id': 9, 'end_character': 106, 'bleu_score': 1.0, 'section': 'Section::::Common processes.:Black and white negative processing.\n'}]}]