"Poet Immortal" Li Bai was not dazzled, it turns out that he could really "produce purple smoke"!

Recently, many netizens captured a wonderful scene: the sunlight shines on the smoke of incense, and a dreamy purple color appears. This scene of "purple smoke" is a vivid embodiment of the poem " Sunlight shines on the incense burner and produces purple smoke " ("Looking at the Waterfall at Mount Lu") written by the "Poet Immortal" Li Bai more than a thousand years ago. "Looking at the Waterfall at Mount Lu" is one of the ancient poems that primary school students must learn and recite. The reason for the "purple smoke" in the poem is often skipped by teachers when teaching because it involves deeper traditional cultural knowledge and physics knowledge. Some teachers even explain that it should be white smoke, and Li Bai may have seen it as purple because he was dazzled by the waterfall for too long.

Netizens took real photos of the "purple smoke" phenomenon (video screenshot, source see watermark)

In fact, this poem was written by Li Bai after careful observation. The scene of "purple smoke" in the poem is not only in line with scientific facts, but also has profound cultural connotations. To explain all this, we must first talk about the propagation of light.

Rayleigh scattering blue sky

Anyone who has studied junior high school physics knows that sunlight propagates in a straight line when passing through the same uniform medium, and the direction of propagation changes when passing through the same uniform medium. Therefore, when sunlight enters the atmosphere, which has a complex and uneven composition, it collides with air molecules, dust, aerosols and other media, resulting in scattering. Scattering is also an optical phenomenon, just like direct radiation, reflection, refraction, and diffraction.

The scattering of sunlight in the atmosphere can be divided into two situations: when it encounters air molecules with a diameter smaller than the wavelength of sunlight, it will be selectively scattered, and its scattering intensity is related to the wavelength. This scattering is called molecular scattering; when it encounters dust, aerosol (a general term for solid or liquid particles suspended in the atmosphere, such as fog, smoke, and haze) and other substances with a diameter larger than the wavelength, light waves within a certain range will all be scattered (that is, non-selective scattering), and this scattering is called Mie scattering.

Schematic diagram of light scattering (video screenshot)

In molecular scattering, the scattering intensity is inversely proportional to the fourth power of the wavelength of light. In other words, the shorter the wavelength of light, the stronger the scattering, and vice versa. This law was discovered by British physicist Rayleigh in the early 20th century, so molecular scattering is also called Rayleigh scattering.

Rayleigh scattering can make the sky appear in different colors. As we all know, sunlight is a complex light composed of seven monochromatic lights: red, orange, yellow, green, blue, indigo, and violet. The longest wavelength is red light, usually about 650nm (nanometers); the shortest wavelength is violet light, usually about 400nm (nanometers), and blue light is second, usually 450nm (nanometers). Due to the influence of Rayleigh scattering, when sunlight passes through the atmosphere, the red light has a longer wavelength and less scattering, so it has stronger penetration and can usually penetrate the atmosphere to the ground; while violet and blue light have shorter wavelengths and more scattering, so their penetration is very weak. In the cool autumn or after rain, there is less dust and aerosols in the atmosphere. Under the strong Rayleigh scattering of atmospheric molecules, the blue and violet light of sunlight is scattered all around and even covers the sky. The human eye is more sensitive to blue and less sensitive to purple, so the sky looks blue.

Sunlight spectrum (Source: China Science Network)

Let's take an example to explain the Rayleigh scattering of red light and blue-violet light. Suppose a 2.0-meter-tall red light and a 1.5-meter-tall blue-violet light start from space at the same time and run towards the earth. When passing through the atmosphere full of "stones" about 0.5 meters high, the red light has long legs and can easily jump over; the red-violet light has short legs and can only change direction to run. Because there are too many "stones", the red-violet light has to change direction many times, and finally it feels like it fills the sky.

Light scattering intensity curve (Source: China Science Popularization Network)

It is worth mentioning that Rayleigh scattering is also used in traffic lights. Red light is used as a traffic light because in foggy weather, red light can penetrate the fog and is easier to be seen by drivers or pedestrians, thus reducing the occurrence of accidents.

The reason for the poem is analyzed based on facts

Rayleigh scattering makes the sky look blue. Is this phenomenon related to the phenomenon of "sunlight shining on incense burners producing purple smoke"?

This needs to be explained in conjunction with the scene described in "Viewing the Waterfall at Mount Lu".

The "incense burner" in the poem refers to the Xianglu Peak in the northwest of Lushan Mountain, which is named because of its round top, which looks like a huge incense burner. In the early morning, under the shining golden sunlight, bursts of purple clouds rise from the Xianglu Peak, like smoke, which is very beautiful.

From this, we can draw the following three conclusions: First, the "purple smoke" is not smoke, but a purple haze, exactly the same as the purple smoke of incense photographed by netizens; second, the weather was very good that day, with few clouds and fog, and few particulate media in the air, so Rayleigh scattering occurred. If it was foggy or cloudy that day, and there were more and larger particulate media in the air, then the sunlight would appear white due to Mie scattering, and the poem could only be "the sun shines on the incense burner and produces white smoke"; third, because the Xianglu Peak is towering into the clouds and the terrain is complex, airflow is prone to form vortices or turbulence, and when sunlight shines into them, it is constantly scattered, further enhancing the purple smoke effect and making it easy for people to observe.

Ink painting "View of Lushan Waterfall" (Source: Chinese Style Animation Network)

Some people may ask, since Rayleigh scattering makes the Xianglu Peak present blue and purple light with shorter wavelengths, and the human eye is more sensitive to blue light, why didn't Li Bai write "the sun shines on the incense burner and produces blue smoke"? The answer is mainly two aspects. First, it is very likely that when Li Bai was watching the waterfall, due to the special geographical environment and climatic conditions, the diameter of the air flow molecules on the Xianglu Peak was similar to the wavelength of purple light, so the purple light was the strongest in the scattered light, scattered the most, and was easy to be seen. Second, compared to the sky, the Xianglu Peak is closer to the observer, so it is easier to see purple light. The netizens took close-up photos of the incense smoke dominated by purple, which is also the reason.

Inheriting culture and blending emotions

Although there is a reasonable scientific explanation for "the sun shining on the incense burner and producing purple smoke", and there are also netizens taking photos to prove it, many people who have been to Lushan and seen the Lushan Waterfall have never seen the "purple smoke". So why did Li Bai write about this extremely rare scene in his poem?

This has to start with Li Bai's personality and traditional Chinese culture. One of the reasons why Li Bai is known as the "Poet Immortal" is that he is a Taoist who pursues a life like an immortal. This pursuit is reflected in many of his poems. Purple is the most respected auspicious color in Taoism, and the idiom "Purple Air Coming from the East" is a clear proof of this. According to legend, during the Spring and Autumn Period, Yin Xi, the governor of Hangu Pass, observed the sky at night and saw purple air coming from the east. Soon, Laozi (Li Er), the founder of Taoism, rode a green ox to Hangu Pass. Later, people used "Purple Air Coming from the East" to describe auspicious signs, and it is still used today; "Purple Smoke" is used to represent purple auspicious clouds. For example, the famous "Traveling to the Immortal Poems" (by Guo Pu of the Eastern Jin Dynasty) contains the sentence "Red pine stands on the upper reaches, and riding a wild goose rides on purple smoke."

Laozi's "Purple Air Coming from the East" arrived at Hangu Pass (Source: Chinese Painting Network)

Li Bai, who was pursuing immortality, looked at the Lushan Waterfall from afar and saw the top of the Xianglu Peak was filled with smoke, which seemed like a fairyland, and inspired him to write a poem. For him, whether it was blue smoke, green smoke or other smoke, since it was fairy air, it was appropriate to use "purple smoke", not to mention that after careful observation, he really saw purple smoke.

From this, we can see that "the sun shines on the incense burner and produces purple smoke" cleverly borrows natural phenomena to explain traditional culture, making this scene a model of the combination of science and literature. This also tells us that science is not necessarily boring, but also full of poetry and beauty.

The talented Li Bai was not only not dazzled, but observed very carefully and wrote it down in his poem. More than a thousand years later, through the fact that netizens took close-up photos of purple incense smoke, we not only have a deeper understanding of the essence of "the sun shining on the incense burner to produce purple smoke", but also feel the beauty of the fusion of science and art.

References:

1. Paper "On the "Fake" Theory of Li Bai's Quatrain "Looking at the Waterfall at Mount Lu", author Shao Jie, "Yindu Journal" No. 1, 2018

2. Paper "Evaluation of the Competitiveness of Lushan Cultural Heritage Tourism", author Sun Jin, "Chinese Geology" 2020 No. 2

3. Paper "Light Scattering Phenomenon and Research Progress", Journal of Radio Science, Issue 5, 2018

Author: Wei Deyong

Member of Guangdong Shenzhen Writers Association