What is a mirage?

In calm seas, rivers, lakes, snowfields, deserts or Gobi, occasionally towering towers, city walls and trees appear in the air or "underground", which is called a mirage. This illusion often appears on the sea surface of Penglai, China. The ancients attributed it to the dragon, exhaling and forming the tower wall, hence the name. Mirages often appear on the sea and in the desert. Mirage is the result of light refraction along a straight line in gas layers with different densities. There are many kinds of mirage, which can be divided into Shanghai mirage, Xiahai mirage and seaside mirage according to their position relative to the original objects. According to its symmetrical relationship with the original object, it can be divided into positive, lateral, positive and negative; According to the color, it can be divided into colored mirage and colorless mirage.

trait

Mirages have two characteristics: first, they appear repeatedly in the same place, for example, mirages often appear over Alaska in the United States; The second is simultaneous appearance. For example, the mirage in Penglai, China often appears in May and June every year, the mirage near Zimlensk, Russia often appears in spring, and the mirage in Alaska, USA generally appears within 20 days after June 20th.

nature

Mirages have been concerned by the world since ancient times. In western mythology, a mirage is depicted as the incarnation of the devil, a harbinger of death and misfortune. In ancient China, a mirage was regarded as a fairyland. Qin Shihuang, Emperor Wudi and others led people to Penglai to seek immortality, and also sent people to Penglai for a panacea many times. Modern science has correctly explained most mirage, which is considered as a virtual image formed by the refraction of light reflected by objects on the earth through the atmosphere. The so-called mirage is an optical illusion.

Formation condition

Mirages are closely related to the geographical location, geophysical conditions and meteorological characteristics of those places at a specific time. The abnormal distribution of temperature is the meteorological condition for the formation of most mirage. Take the formation of summer flies as an example. In summer, when the sun burns in the desert, the sand is burnt. Because of the small specific heat of sand, the temperature of low-level air near sand rises rapidly, while the temperature of high-level air is still very low, thus forming an abnormal distribution of temperature. Due to thermal expansion and contraction, the density of hot air in the lower layer near the sand is small, and the density of cold air in the upper layer is large, so that the refractive index of air is small and the air in the upper layer is large. When the light reflected by a distant higher object enters the sparse air in the lower layer from the dense air in the upper layer, it is constantly refracted, and its incident angle gradually increases, which is equal to the critical angle, and total reflection occurs. At this time, if people look against the reflected light, they will see the cockroaches below. Because of the dark color of asphalt pavement, it has strong absorption ability in the hot summer sun, and it will also form the distribution characteristics of cold and dense upper air and hot and dense lower air, so it will also form underflow.

A mirage in the desert is the refraction of sunlight when it meets air with different densities. In the desert, the sand is 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 air, when there is no wind, the vertical temperature difference over the desert is very significant, with high air density in the upper layer and low air density in the lower layer. When sunlight enters the low-density air layer from the high-density air layer, the speed of light changes, and after the refraction of light, the oasis in the distance appears in front of people. On the sea or river, this kind of "mirage" phenomenon happens from time to time. A mirage is an optical illusion, a virtual image formed by the refraction of light reflected by objects on the earth through the atmosphere. Phantom is a mirage for short. According to the principle of physics, a mirage is because different air layers have different densities, and the refractive index of light in air with different densities is different. That is, because the density between warm air on the sea surface and cold air in the high air is different, it refracts light. Mirages are closely related to the geographical location, geophysical conditions and meteorological characteristics of those places at a specific time. The abnormal distribution of temperature is the meteorological condition for the formation of most mirage.

cause

Sailing on the calm sea or overlooking the seaside, you will often see images of distant ships, islands or towers reflected in the air; People who travel in the desert sometimes suddenly find a lake in the desert in the distance, and the shadow on the lake is swaying, which is desirable. But a gust of wind came and these scenes suddenly disappeared. It turns out that this is a mirage, commonly known as a mirage, or a mirage for short. Phantom diagram

Why is this happening? To answer this question, we must start with the refraction of light. When light propagates in a uniform medium with the same density, the speed of light is constant, and it propagates in a straight line. However, when light obliquely enters another medium with different density, the speed of light will change and the propagation direction will be tortuous. This phenomenon is called refraction. When you insert a straight rod obliquely into the water, you can see that the underwater part of the rod and its exposed part seem to be broken, which is the result of light refraction. Someone once used a device to project light from water to the interface between water and air. You can see that light is divided into two parts at this interface: one part is reflected into water and the other part is refracted into air. If the mirror in the water is rotated to make the light incident on the interface more inclined, then the refraction of light in the air will be more intense. When the light incident on the interface is shown in the figure on the left, all the light is reflected into the water, and no light is refracted into the air. This phenomenon is called total reflection. Air itself is not a homogeneous medium. Generally speaking, its density decreases with the increase of height. The higher the height, the smaller the density. When light passes through the air layer at different heights, it will always cause some refraction, but this kind of refraction phenomenon has become accustomed to our daily life, so we don't feel anything strange. In summer, the humidity of seawater is relatively low during the day, especially on the sea surface where cold water flows, and the water temperature is even lower. The low-level air is affected by the water temperature, which is colder than the high-level air, resulting in the abnormal phenomenon of cooling and heating (under normal circumstances, the average temperature drops by about 0.6℃ for every rise of 100m). Because the air pressure in the lower air layer is very high, and now the temperature is lower than that in the upper air layer, the difference between density and rareness under the air layer is extremely significant. If there is a ship under our eastern horizon, it is usually invisible. If the difference between dense air and thin air is too big at this time, the light from the ship will gradually refract from the dense air layer to the thin air layer, and then return to the dense air layer below after total reflection in the upper layer; After such a curve, it finally enters our eyes and we can see its image. Because people's vision always feels that objects come from a straight line, the ship image we see is much higher than the real thing, so it is called a mirage. There is a miaodao islands in Bohai Sea, China. In summer, the sea water temperature is low during the day, and the air density will be significantly different between underground density and sparse density. In Penglai County (called Dengzhou in ancient times) on the south bank of Bohai Sea, you can often see the phantom of miaodao islands. In the Song Dynasty, Shen Kuo recorded in his famous "Meng Qian Bi Tan": "There are clouds in the sea of Dengzhou, such as temples, city maps, chariots and horses, which are vivid in my mind." This is a mirage he saw in Penglai. 1624 (May 2, 20041day) On July 6, a mirage occurred in Dengzhou, lasting more than 7 hours. Yuan Keli, the outgoing governor of Denglai, witnessed the whole process and was very excited. A poem "Poems on Haicheng" recorded the grand occasion at that time in detail, with beautiful words, which can make people who have never seen Haicheng feel immersive. 1In the early morning of May 22, 933 1 1 point, a mirage was also found on Zhucha Island in Qianhai, Qingdao (the outer mouth of Jiaozhou Bay), which spread all over the city for a time and many people went to watch it. 1975, a 6-hour mirage appeared in the sea near Guangdong province. Mirages can be seen not only on the sea in summer, but also on the river sometimes. For example, 1934 appeared on the river near Nantong on August 2. It was cold that day, and it was particularly hot. In the afternoon, I suddenly found balcony walls and tree houses over the Yangtze River, both of which are more than 20 miles long. About half an hour later, it moved eastward and suddenly disappeared. Then three mountains appeared, towering into the sky, with a mountain in the middle, much like a censer; It took half an hour to disappear. In the desert, the sand is scorched by the sun during the day, and the temperature near the sand layer rises very quickly. Because air is not good at heat transfer, when there is no wind, the heat exchange between the upper and lower air is very small, which makes the vertical difference between cold and hot temperatures very significant, leading to the abnormal phenomenon that the density of the lower air is less than that of the upper air. In this case, if there is a tree in front of it and it grows in a relatively humid place, the light obliquely projected downward from the top of the tree will be refracted because it enters the air layer with high density. When the refracted light reaches the hot and thin air layer near the ground, total reflection occurs, and the light is reflected from the low-density air layer near the ground back to the higher-density air layer above. In this way, after a concave arc light, the image of the tree is sent to people's eyes, and the reflection of the tree appears. Because the reflection is below the real thing, it is also called a mirage. This kind of reflection can easily give people the illusion that there are trees by the water, thinking that there must be a lake in the distance. Most people who have traveled in the desert have had similar experiences. A photographer who filmed the film "Climbing Mount Everest in Hishapang" saw such a mirage while walking on a vast arid grassland. He ran in the direction of a mirage, trying to fetch water for cooking. When he ran there and saw that there was no water, he found himself cheated by a mirage. This is because hay, like sand, can be heated by the scorching sun, which gradually increases the density of the air layer from bottom to top, thus creating a mirage. No matter what kind of mirage, it can only appear in windless or extremely weak weather conditions. When strong winds combine, which causes the agitation and mixing of the upper and lower air, the air density difference between the upper and lower air decreases, and there is no abnormal refraction and total reflection of light, then all illusions disappear immediately. In a word, this is a very interesting refraction phenomenon.

A good place to watch

According to the data, Long Island is the most frequent mirage area in China, especially after the rain in July and August. Long Island, known as miaodao islands in ancient times, also known as Changshan Islands, consists of 32 islands, with a land area of 56 square kilometers, a sea area of 8,700 square kilometers and a coastline of 146 kilometers. It is the only island county in Shandong Province and belongs to Yantai City. The county has jurisdiction over 2 towns and 8 townships with a population of 45,000. The county seat is located in Nanchangshan Island. Long Island belongs to the continental climate of East Asian monsoon region, with the characteristics of warm winter and cool summer. The annual average temperature is 1 1.9℃, and the frost-free period is 243 days. The forest coverage rate of the county is 53.2%, and its unique geographical location and superior natural conditions make it the only place for migratory birds to migrate. Every year, more than 200 species of migratory birds pass by, with a number of millions, enjoying the reputation of "post station" for migratory birds and being listed as a national nature reserve. Long Island, called miaodao islands in Chinese territory (called Shamen Island in ancient times), is located between Shandong Peninsula and Liaodong Peninsula, at the intersection of Yellow Sea and Bohai Sea, and is the only island county in Shandong Province. * * * consists of 32 islands with a total area of 56 square kilometers. The total length of coastline 146 km, with a permanent population of 45,000.

Long Island is the blessing of the four seas in China. There are four myths and folklore figures: "Mazu protects the sea", "Eight Immortals cross the sea", "Changyu cooks the sea" and "Jingwei fills the sea". Going to Long Island, you can eat hundreds of kinds of seafood, live in a fisherman's kang, visit the island scenery, buy desirable specialties and entertain folk customs. The mountains on Long Island are "the beauty of Wuling Shili Gallery", and the sea on Long Island is "the beauty on both sides of Lijiang River". "The East China Sea is rich in water and fish, and the South China Sea is rich in Na Xiang's beautiful fruit country", "The Yellow Sea sets sail at dusk, and the fishing boats in the Bohai Sea sing late" are the most simple, bold, true and vivid portrayal of the life of fishermen in Jing Xian. Long Island is a blessed land of China, with the reputation of "the hometown of abalone, scallop and kelp in China". Long Island is the Hawaiian island of China, and has the title of "National Nature Reserve, National Scenic Area, National Forest Park, Provincial Seal Nature Reserve and Provincial Geopark". Long Island is a primitive, natural, unprocessed and unique tourist virgin land. Here, there is an island miracle and a beautiful scene. All kinds of strange reefs and rocks formed by marine erosion landforms are simple, quiet and exquisite. In 2003, Long Island Tourism Bureau created "Fairy Mountain on the Sea", "Well-off Fisherman" and "Fisherman's Sea Tour", and paid attention to the management of this brand, integrated tourism resources, and created a green, safe and healthy tourist destination, especially Mazu Stone, which is a blessed land and a happy place and has become a magnificent flower in the island folk culture of our country. The sea fairy mountain is inlaid with a group of islands as green as jewels. This is the beautiful archipelago-miaodao islands, also known as Long Island, which is praised by the world as "the fairy mountain on the sea". Long Island is located in the Bohai Strait between Liaodong Peninsula and Shandong Peninsula, facing the ancient city of Dengzhou (Penglai) across the sea. According to Records of the Historian, Qin and Emperor Wu of the Han Dynasty took pains to rest their horses at the side of Ya Dan Mountain in Penglai, looking at the sea fairy mountain and seeking longevity. This fairy mountain refers to Long Island. Ceng Yun, a Tang poet, said, "Then he heard a magical island on the sea, and the mountain was in nothingness ..." Su Dongpo, a great writer in the Song Dynasty, once looked at the Changshan Islands and could not help but exclaim: "Real immortals live here!" The Journey to the West's Fairy Tales such as "The Flower in the Mirror" describe this place as an illusory paradise. The phantom car in nature is running on the highway, the glistening sunshine is baking the road, only the roar of the wind and the roar of the motor can be heard, and the thinking seems to have stopped. Suddenly, a pool of water appeared not far ahead, and the cars driving in front also reflected clearly in the water. But as the car drove, the beach water was always ahead and finally disappeared slowly. This is a phenomenon that can often be seen on highways. There used to be such a story. Once upon a time, there was a camel team that hesitated in the smoke-filled desert. The hot summer and dry weather make travelers tired, the water in their purses has been drained and their lips are chapped. How they want to drink a mouthful of cold water! Suddenly, in the distant desert ahead, a lake appeared, and palaces and temples on both sides of the lake soared into the sky, which brought them great hope and a cool feeling, so they quickly ran forward. Passing through one sand dune after another, lakes, palaces and temples are still far away, flickering. After a while, suddenly the lake, palaces and temples disappeared without a trace. In a calm sea, river, lake, snowfield, desert or Gobi, occasionally there are visions such as towers, city walls and trees in the air or "underground". We call this phenomenon a mirage. This illusion often appears on the sea surface of Penglai, China. In ancient legends in China, storks were considered to be dragons, and they could exhale to form tower walls. It is also said that Haicheng is the residence of Poseidon, which is located in the "illusory space", hence its name. In Song Dynasty, Shen Kuo wrote in "Talk about Meng Xi": "There are clouds in the sea of Dengzhou, which are clearly visible as palaces, city figures, chariots and horses." In order to illustrate this phenomenon, let's do a "mirage" experiment: in an unventilated room, a smooth iron sheet with a length of 1.5 m and a width of 20 cm is placed horizontally on several small columns made of iron pipes (or wooden sticks instead), and a thin layer of sand is sprinkled on the iron sheet to make a desert surface. Cut the dark paper into trees and camels, stick it on a frosted glass (ivory glass), and put the glass plate at one end of the iron sheet, perpendicular to the iron sheet, so that the trees and camels are exposed to the sand layer. Under the back of the glass plate, a flashlight shines upward, and at the other end of the iron plate, it seems that the bright sky is set off behind the trees and camels. Then, use three small briquette furnaces and heat them under the iron sheet (or use a long charcoal basin, and it is ideal to heat them with a long electric furnace if conditions permit). When heating, pay attention to the uniform heating of the iron sheet everywhere, especially near two-thirds of one end of the ground glass. After heating for a certain period of time, when you get close to the sand surface with your hands and feel very hot, start to observe the ground glass along the thin iron sheet. You can find the reflection of trees and camels under the sand, just like the reflection formed when there is a lake next to them. This phenomenon is a "mirage". In hot summer, sometimes you can see the reflection of houses and trees on the asphalt road. This is essentially a "mirage" phenomenon. How is a mirage formed in nature? Mirage is an abnormal refraction phenomenon, which is the result of light propagation and refraction in the atmosphere with different vertical densities. It is often divided into upward, downward and sideways mirage. Any image or illusion that seems to reflect from the air layer in the sky is called a mirage. Mirages in the world often appear at sea and in places covered with snow and ice in the north. This is because the surface consumes heat when seawater evaporates, and the temperature of seawater rises slowly. In areas covered with snow and ice, because the snow and ice can reflect most of the sunlight and consume a lot of heat, the temperature in the lower layer becomes very low, so these places are most prone to strong inversion. If there is a strong inversion in the near-surface layer, the air density will decrease rapidly with the height, and light will spread in the air layer where the temperature increases with the height, so that the air density will decrease sharply with the height and bend downward. The light emitted by buildings on the distant horizon will enter the eyes of observers after refraction, and a mirage will appear. Any image or illusion that looks like an object reflected from the ground is called a mirage. Mirages often appear in the desert in hot season or on the sea surface with warm ocean currents in winter. On a sunny day, it is partly cloudy and windy. When the sun shines on the dry sand, the specific heat of the sand is small and the soil temperature rises rapidly. There is almost no water evaporation here, and the heat transfer of soil molecules is extremely slow, and the heat is concentrated in the surface layer, so the air temperature near the soil layer also rises rapidly, but the upper air is still very cold. When the near-surface layer is a strong cooling layer, the temperature decreases rapidly with height, and the air density increases rapidly with height. When light propagates in a gas layer whose temperature decreases with height, it will bend upward. The light from the distant horizon will be refracted and directly enter the observer's eyes, and a mirage will appear.