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.