Mengxi’s Written Talk Mirage

Mirages in the desert

On calm seas, rivers, lakes, snowfields, deserts or Gobi, tall buildings and city outlines occasionally appear in the air or "underground" , trees and other illusions are called mirages. This kind of illusion often appears on the sea in Penglai, Shandong, my country. The ancients attributed it to a mirage belonging to the dragon family, which breathed out its breath to form a towering city outline, hence its name. Mirages often occur at sea and in deserts. A mirage is the result of light refracting in air layers with different densities along a straight line. There are many types of mirage. According to the position of its appearance relative to the original object, it can be divided into superior mirage, inferior mirage and lateral mirage; according to its symmetrical relationship with the original object, it can be divided into positive mirage, lateral mirage and lateral mirage. And reverse mirage; according to color, it can be divided into chromatic mirage and achromatic mirage, etc.

Characteristics

Mirages have two characteristics: first, they appear repeatedly in the same place, such as mirages often appear over Alaska in the United States; second, they appear at the same time, such as in my country Most mirages in Penglai appear in May and June every year, mirages near Chimlensk in Russia often appear in spring, and mirages in Alaska in the United States generally appear within 20 days after June 20.

Essence

Since ancient times, mirages have attracted the attention of the world. In Western mythology, mirages are depicted as the incarnation of the devil and a bad omen of death and misfortune. In ancient my country, mirages were regarded as fairyland. Qin Shihuang and Emperor Wu of Han once led people to Penglai to search for fairyland, and they also sent people to Penglai many times to seek panacea. Modern science has correctly explained most mirages and believes that mirages are virtual images formed by light reflected by objects on the earth and refracted by the atmosphere. The so-called mirage is an optical illusion.

Conditions for formation

Mirage is closely related to geographical location, geophysical conditions and meteorological characteristics of those places at a specific time. Abnormal distribution of temperature is the meteorological condition that causes most mirages. Take the formation of summer mirages as an example. In the summer, the sun is blazing in the desert, and the sand is scorched. Because the specific heat of the sand is small, the temperature rises very quickly. The temperature of the lower air near the sand rises very high, while the temperature of the upper air remains very low, thus forming a temperature pattern. Abnormal distribution. Due to thermal expansion and contraction, the density of hot air in the lower layer close to the sand is small and the density of cold air in the upper layer is high. In this way, the refractive index of the air is small in the lower layer and large in the upper layer. When the light reflected from a taller object in the distance is continuously refracted from the denser air in the upper layer into the sparser air in the lower layer, its incident angle gradually increases, and total reflection occurs when it increases to equal the critical angle. At this time, if a person walks backwards If you look at the reflected light, you will see a mirage. Due to the dark color of the asphalt road surface and its strong absorption capacity under the scorching sunshine in summer, it will also form a distribution pattern of cold and dense air in the upper layer above the road surface, while hot and low density air in the lower layer, so mirages will also be formed.

The "mirage" that occurs in the desert is the refraction of sunlight when it encounters air of different densities. In the desert, the sand and rocks are baked by the sun during the day, and the temperature on the surface of the sand layer rises rapidly. Due to the poor heat transfer performance of the air, when there is no wind, the vertical temperature difference over the desert is very significant. It is hot at the bottom and cold at the top. The air density in the upper layer is high and the air density in the lower layer is low. When sunlight passes from a high-density air layer into a low-density air layer, the speed of light changes. After the refraction of light, the oasis in the distance appears in front of people's eyes. This "mirage" phenomenon sometimes appears on the sea or river. A mirage is an optical illusion, a virtual image formed by the refraction of light reflected by objects on the earth through the atmosphere. Mirage is referred to as mirage. According to the principles of physics, mirage is caused by different air layers having different densities, and light has different refractive index in air with different densities. It is caused by the refraction of light due to the difference in density between the warm air on the sea surface and the cold air at high altitudes. Mirages are closely related to geographical location, geophysical conditions, and meteorological characteristics of those places at specific times. Abnormal distribution of temperature is the meteorological condition that causes most mirages.

Causes

When sailing on calm and windless seas or looking at the seaside, you will often see images of distant ships, islands or city towers reflected in the sky; traveling in the desert People sometimes suddenly discover that there is a lake in the remote desert, with the swaying shadows of the trees on the lakeside, making people yearn for it. But when the strong wind blows, these scenes suddenly disappear. It turns out that this is an illusion, commonly known as a mirage, or simply a mirage. Schematic diagram of mirage

Why does this phenomenon occur? To answer this question, we must first start with the refraction of light. When light travels in a uniform medium of the same density, the speed of light remains unchanged and it advances in a straight direction. However, when light enters another medium of different densities obliquely from this medium, the speed of light will change. It changes and the direction of progress also twists and turns. This phenomenon is called refraction. When you insert a straight rod into the water at an angle, you can see that the underwater part of the rod and the part exposed on the water seem to be broken. This is caused by the refraction of light. Someone has used a device to make the light from When the water is projected onto the interface between water and air, you can see that the light is divided into two parts at this interface: one part is reflected into the water, and the other part is refracted into the air. If you turn the mirror in the water so that the light hitting the interface is more tilted, the refraction of the light in the air will appear more severe.

When the light directed toward the interface is as shown in the picture below on the left, all the light is reflected into the water, and there is no more light refracted into the air. This phenomenon is called total reflection. Air itself is not a uniform medium. Under normal circumstances, its density decreases with height. The higher the height, the smaller the density. When light passes through air layers at different heights, it will always cause some refraction, but we are used to this refraction phenomenon in our daily lives, so we don't think it's anything strange. In summer, the humidity of seawater is relatively low during the day, especially when cold water flows through the sea surface. The water temperature is lower. The air in the lower layer is affected by the water temperature. The air in the lower layer is affected by the water temperature and is colder than the air in the upper layer. The abnormal phenomenon of cold at the bottom and warm at the top occurs ( Normal conditions are warm at the bottom and cool at the top. On average, the temperature drops by about 0.6°C for every 100 meters of elevation.) The air in the lower layer is already denser due to higher air pressure. Now coupled with the fact that the temperature is lower than that in the upper layer, the density becomes particularly high. Therefore, the difference between the denser air layer at the lower level and the thinner air layer at the upper level is extremely significant. If there is a ship below our eastern horizon, it is normally invisible. If the difference between the lower air and the upper air is too great, the light from the ship will first be gradually refracted from the dense air layer into the thin air layer, and will undergo total reflection in the upper layer, and then refract back to the lower dense air layer. ; After such a curved line, and finally put into our eyes, we can see its image. Since human vision always perceives objects as coming from a straight direction, the image of the ship we see is much higher than the actual object, so it is called a mirage. There is a temple island archipelago in the Bohai Sea of ??my country. In summer, the seawater temperature is low during the day, and the air density will be significantly different from dense underground to thin. In Penglai County (also called Dengzhou in ancient times) on the south coast of the Bohai Sea, temples can often be seen. The phantom of the island archipelago. Shen Kuo of the Song Dynasty recorded this in his famous book "Mengxi Bi Tan": "There are clouds in the sea of ??Dengzhou from time to time, such as palaces and terraces, figures on the battlements, chariots and horses, and crowns, which can be clearly seen." This is What he saw in Penglai was a mirage. On July 6, 1624 AD (May 21, the fourth year of tomorrow's Qi Dynasty), a mirage occurred on the sea surface of Dengzhou that lasted for more than seven hours. Yuan Keli, the outgoing governor of Denglai at that time, witnessed the whole process and felt very sad. Excited, a "Hai Shi Poem" recorded the grand occasion in detail at that time. The beautiful diction can make people who have never seen the Hai City feel like they were there. Coupled with Dong Qichang's calligraphy, it became a legend of the East China Sea. Immortal work. At around 11 o'clock in the morning on May 22, 1933, a mirage was also discovered on Zhucha Island in Qianhai (outside of Jiaozhou Bay), Qingdao. It spread to the whole city and many people went to watch it. In 1975, a mirage lasted for six hours on the sea near Guangdong Province. Not only can mirages be seen on the sea in summer, they can also sometimes be seen on the river. For example, it appeared on the river near Nantong on August 2, 1934. It was a sweltering day and the weather was extremely hot. In the afternoon, I suddenly noticed towers, city outlines, trees and houses reflected in the sky above the Yangtze River. The entire mirage was more than 20 miles long. About half an hour later, it moved eastward and suddenly disappeared. Then three more mountains appeared, towering into the clouds, with a mountain in the middle that looked like an incense burner. It took another half an hour before they all disappeared. In the desert, the sand and gravel are scorched by the sun during the day, and the temperature near the sand layer rises very quickly. Since air is not good at heat transfer, when there is no wind, the heat exchange between the upper and lower layers of air is very small, which makes the vertical difference in temperature between hot and cold at the bottom very significant, and leads to the abnormal phenomenon that the air density in the lower layer is smaller than that in the upper layer. In this case, if there is a tree in front of you and it grows in a relatively humid place, then the light projected downward from the treetop will pass from the denser air layer into the less dense air layer. refraction. When the refracted light reaches the hot and thin air layer close to the ground, total reflection occurs. The light is reflected from the less dense air layer near the ground back to the denser air layer above. In this way, through a downward and downward curved light, the image of the tree is sent to people's eyes, and a reflection of the tree appears. Because the reflection is located below the real object, it is also called a mirage. This kind of reflection can easily give people the illusion of the shadow of trees by the water, thinking that there must be a lake in the distance. Most people who have traveled in the desert have similar experiences. A photographer who shot the film "Climbing Shishapangma" once saw such a mirage while walking on a vast dry grassland. He ran in the direction of the mirage to get some water to cook. meal. When he ran there and saw there was no water source, he realized that he had been fooled by a mirage. This is because dry grass, like sand, can be heated up by the scorching sun, causing the density of the air layer to gradually increase from bottom to top, thus producing a downward mirage. No matter what kind of mirage, it can only appear in weather conditions with no wind or very weak wind. When strong winds come together, causing the agitation and mixing of the upper and lower air, the density difference between the upper and lower air is reduced, and there is no abnormal refraction or total reflection of light, then all the illusions disappear immediately. All in all, this is a very interesting refraction phenomenon.