The setting sun in the long river is a phenomenon of light refraction.

Refraction occurs when light passes from one medium into another. At sunset, sunlight slants through the uneven atmosphere, which can be regarded as passing through two media with different refractive indexes. Although the refractive index is not very high, the light path of oblique sunlight is long enough and the incident angle is large enough.

When the setting sun is close to the horizon, the lower end of the sun appears to be above the horizon, but this is only an image produced by refraction. In fact, the lower end of the sun is already below the horizon; the upper end of the sun is also farther than the direct position. Lower. The refracted images of the upper and lower ends of the sun are displaced, and the length of the vertical axis is significantly shorter than the horizontal axis. So the sunset is mostly flat, like this.

Explanation of the phenomenon of "sunset sun in the long river"

If what Wang Wei saw was a "half-set sun", since half of it is below the horizon, the difference in the upper and lower refraction angles would be only "complete" According to people's inherent impression, the sun is round, and it is difficult to judge whether the sunset is a "big half sunset" or a "small half sunset", so people will use the sun in their memory to interpret the missing sun in their minds. After filling in the half, the "Sunset over the Long River" is complete.

But in fact, there is another possibility for "sunset yen on the long river". Here is a quote from Yan Shuangying: "The eyes are the most unreliable organ in the human body. Your eyes will deceive you." It is very possible that the setting sun is still flat, and Wang Wei was just deceived by his own eyes.

Wearing vertical striped clothes makes you look thinner, while horizontal striped clothes make you look fatter. This is because the complex pattern of stripes affects the neural integration coding in two advanced brain areas of primates that encode visual movement information. mechanism to make the vertical stripes look "longer".

The previous sentence of "The sun sets over the long river" is "The solitary smoke is straight in the desert." The "solitary smoke is straight" in this sentence probably plays the role of vertical stripes, making the sunset look longer. ".

In addition, there may be more complex factors affecting whether the "Long River Sunset" is round or not. "Envoy to the Fortress" was composed in the Gobi Desert. The temperature difference between morning and evening is large in the desert, and it is unknown what the temperature will be at sunset. If the temperature is too high and air molecules move violently, the sunset may appear distorted.