Recently, many netizens filmed a wonderful scene: the smoke of incense shining in the sun appeared dreamy purple. This scene of "purple smoke" is a vivid embodiment of the poem "Purple Smoke from the Rizhao Incense Burner" ("Wanglu Mountain Waterfall") by the "poet immortal" Li Bai more than 1,000 years ago. "Wanglu Mountain Waterfall" is one of the ancient poems that primary school students must learn and memorize, the reason for the poem about "raw purple smoke", because it involves deep traditional cultural knowledge and physical knowledge, teachers often skip it when teaching, and even some teachers explain that it should be white smoke, Li Bai may be dazzled by the waterfall for too long because of purple.
Netizens took a real shot of the phenomenon of "raw purple smoke" (screenshot of the video, see watermark for the source)
In fact, this poem was written by Li Bai after careful observation, and 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 have to start with the propagation of light.
Rayleigh scatters blue skies
Anyone who has studied physics in junior high school knows that sunlight travels in a straight line through the same homogeneous medium, and the direction of propagation changes when it passes through the same homogeneous medium. As a result, sunlight enters the complex and uneven atmosphere and collides with air molecules, dust, aerosols, and other media, resulting in scattering. Scattering is also an optical phenomenon like direct light, reflection, refraction, diffraction, etc.
The scattering of sunlight in the atmosphere is divided into two situations: when it encounters air molecules with a diameter smaller than the wavelength of sunlight, it will carry out selective scattering, and its scattering intensity is related to the wavelength, this kind of scattering is called molecular scattering; when it encounters dust, aerosols (the general term for solid or liquid particles suspended in the atmosphere, such as fog, smoke, haze) and other substances with a diameter larger than the wavelength, a certain range of light waves will be scattered (that is, no selective scattering), this kind of 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 the British physicist Rayleigh in the early 20th century, so molecular scattering is also called Rayleigh scattering.
Rayleigh scattering can make the sky appear different colors. As we all know, sunlight is a compound color light composed of seven monochromatic lights: red, orange, yellow, green, blue, indigo, and violet. The longest wavelength is red light, usually about 650 nm (nanometers), the shortest wavelength is violet light, usually about 400 nm (nanometers), followed by blue light, usually 450 nm (nanometers). Due to the influence of Rayleigh scattering, when sunlight passes through the atmosphere, the red light in it is more penetrating due to its longer wavelength and less scattering, so it can usually penetrate the atmosphere and reach the ground, while violet and blue light are more scattered and weak due to their shorter wavelength. Autumn is crisp or sunny after rain, there is less dust and aerosols in the atmosphere, sunlight is scattered to the surrounding and even full of the sky under the strong Rayleigh scattering of atmospheric molecules, and the human eye is more sensitive to blue, followed by purple feeling, so the sky looks blue.
Sunlight spectrum (Source: China Science Network)
Here's an example to illustrate the Rayleigh scattering of red 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, and when passing through an atmosphere full of "stones" about 0.5 meters high, the red light has long legs and can easily skip it, while the blue-violet light has short legs and can only run in different directions. Because there are so many "stones", the blue-violet light has to change direction many times, and eventually it feels like the sky is all over the place.
Scattering intensity curve of light (Source: China Science Network)
It is worth mentioning that Rayleigh scattering is also used in traffic lights. The red light is used as the indicator light because in foggy weather, the red light can penetrate the fog and be more easily seen by drivers or pedestrians, thus reducing the occurrence of accidents.
The original analysis of the poem is based on facts
Rayleigh scattering makes the sky look blue, is this phenomenon related to the "purple smoke from the sunshine incense burner"?
This has to be explained in conjunction with the scene described in "Wanglu Mountain Waterfall".
The "incense burner" in the poem refers to the incense burner peak in the northwest of Lushan Mountain, which is named because the top of the peak is round and resembles a huge incense burner. In the early morning, under the golden sunlight, the incense burner peak rises with bursts of purple glow, like smoke, which is very beautiful.
The following three conclusions can be drawn: first, the "purple smoke" is not smoke, but a purple glow, which is exactly the same as the incense purple smoke taken by netizens; second, the weather is very good that day, with few clouds and less fog, and less granular medium in the air, so what happens is Rayleigh scattering, if there is fog or cloudiness on the day, and there are more and larger granular media in the air, then the sunlight will appear white due to Michaelis scattering, and the poem can only be "white smoke from the sunshine incense burner"; Third, because the incense burner peak is towering into the clouds, the terrain is complex, and the air flow is easy to form vortices or turbulence, and when the sunlight shines into them, it constantly produces scattering, and the effect of purple smoke is further strengthened, and it is easy to be observed.
Ink painting "Wanglu Mountain Waterfall" (source: Chinese Style Animation Network)
Some people will ask, since Rayleigh scattering makes the peak of the incense burner appear blue light and purple light with shorter wavelengths, and the human eye is more likely to perceive the blue light, why didn't Li Bai write it as "the sunshine incense burner produces blue smoke"? The answer is mainly twofold. First, it is likely that when Li Bai watched the waterfall, due to the special geographical environment and climatic conditions, the diameter of the airflow molecules on the incense burner peak is about the same as the wavelength of purple light, so the purple light is the strongest and scatters the most in the scattered light, and it is easy to be seen. Second, relative to the sky, the incense burner peak is closer to the observer, and it is easier to see the purple light. Netizens photographed purple-dominated incense smoke at close range, which is also the reason.
Inherit culture and blend emotions
Although there is a reasonable scientific explanation for "Purple Smoke from the Rizhao Incense Burner" and there are also photographers from netizens to corroborate it, many people who have been to Lushan and seen the Lushan Waterfall have never seen the scene of "Purple Smoke". So why did Li Bai write about the extremely difficult to see scene into the poem?
This has to start with Li Bai's personality and traditional Chinese culture. One of the reasons why Li Bai is known as a "poet immortal" is that he is a cultivator who pursues a life like a god. This quest is embodied in many poems. Purple is the most auspicious color of Taoism, as evidenced by the idiom "purple qi comes from the east". According to legend, in the Spring and Autumn Period, Hangu Pass ordered Yin Yin to watch the sky at night, and saw that there was purple gas coming from the east, and soon, the founder of Taoism, Lao Tzu (Li Er), really rode a green ox to Hangu Pass. Later, people used "purple qi coming from the east" as a metaphor for auspicious signs, which is still used today, and "purple smoke" is used to express the meaning of purple clouds, such as the famous "Poems of Wandering Immortals" (Eastern Jin Dynasty Guo Pu) has the sentence "Red pine is in the upper reaches, riding the purple smoke".
Lao Tzu's "Purple Qi Comes from the East" to Hanguguan (Source: Chinese Painting Network)
Li Bai, who pursues the fairy road, looks at the Lushan Waterfall from afar, and sees the smoke on the top of the incense burner, which is like a fairyland, and the poetry is prosperous. For him, whether it is blue smoke, green smoke or other smoke, since it is immortal qi, it is appropriate to use "purple smoke", not to mention that he really saw purple smoke after careful observation.
It can be seen from this that "Purple Smoke from the Rizhao Incense Burner" cleverly borrows natural phenomena to explain traditional culture, making this scene a model of the combination of science and literature. It also tells us that science doesn't have to be boring, it's just as poetic and beautiful.
The talented Li Bai was not only not dazzled, but observed very carefully and wrote it into the poem with heart. More than 1,000 years later, through the fact that netizens took pictures of purple incense smoke up close, we not only have a deeper understanding of the essence of "purple smoke from the sunshine incense burner", but also feel the beauty of the fusion of science and art.
Source: WeChat public account "Institute of Physics, Chinese Academy of Sciences"
Author: Wei Deyong
Editor: Wu Xicheng
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