What is the expressive effect of the word "lone sail" in Wangtianmen Mountain?

Looking at Tianmen Mountain, if you come to the place where the poet views the scenery at this time, you will see the majesty of Tianmen Mountain and the scenery of the mighty rushing river, and you will feel the free and unrestrained spiritual outlook.

"Looking at Tianmen Mountain" is a poem written by Li Bai, a great poet of the Tang Dynasty, when he visited Tianmen Mountain on his way to Jiangdong in the 13th year of Kaiyuan (725).

Full text: Tianmen interrupts the opening of Chujiang River, and the clear water flows eastwards until this point. ?The green mountains on both sides of the strait stand out from each other, and a solitary sail comes from the sun.

Translation: Tianmen Mountain broke in the middle because the Chu River broke it open, and the clear water rushed eastwards and turned back here. Towering green mountains on both sides of the river stand opposite each other across the Yangtze River, and a lone boat on the river looks like it is sailing from the sun.

Extended information:

Tianmen Mountain faces the Yangtze River, so Tianmen Mountain cannot be written without the Yangtze River. The first two sentences of the poem are based on the relationship between "river" and "mountain". The first sentence "Tianmen interrupts the opening of the Chujiang River" closely follows the title, always writing about Tianmen Mountain, and emphasizing the majestic momentum of the Chu River flowing eastwards through Tianmen Mountain.

It gives people rich associations: the two mountains of Tianmen were originally a whole, blocking the raging river. Due to the impact of the raging waves of the Chu River, the "Tianmen" was opened, interrupting it and forming two mountains, the east and west.

This poem is all drawn in line, closely following the word "hope" in the title. Every sentence is derived from the word "hope", but the word "hope" is not missing, which shows that its conception is superb. Although the whole poem has only four short sentences and twenty-eight words, it has a broad artistic conception, heroic spirit, harmonious and smooth syllables, vivid and vivid language, and bright colors, which fully demonstrates Li Bai's bold and elegant poetic style.