Having read that passage, he felt a sudden clarity.
Tips
history
From Tao Yuanming's Jin-dynasty 《桃花源记》 (The Peach Blossom Spring): 初极狭,才通人。复行数十步,豁然开朗 - the fisherman squeezes through a narrow opening and emerges into the utopian village. The idiom preserves both senses: a literal wide panorama and the mental 'opening up' that follows.
usage
Works equally for a physical view opening up and for a sudden intellectual insight. Subject is usually the viewer/understander (人 + 豁然开朗) or the scene (眼前/景色 + 豁然开朗).