Phantom problem

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.

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.

However, when the air temperature changes vertically, it will lead to different refraction and total reflection, resulting in a mirage. Mirages appear in different forms because of the different specific conditions of abnormal air density.

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 the denser air and the thinner 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, which will be totally reflected in the upper layer and then return to the dense air layer below; 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 this in his famous Meng Qian Bi Tan:

"When you are in Zhangzhou Sea, there are clouds, such as watching the palace, the people in the city, and the crown of horses and chariots."

This is a mirage he saw in Penglai. 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 downward 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 short, it's interesting and scientific.

[Edit this paragraph] Where is the easiest place to see a mirage?

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 in the 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, see the scenery on the island shore, 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.

Haishang xianshan

On the foggy sea, there are a group of islands as green as jewels. This is the beautiful archipelago known as "Fairy Mountain on the Sea"-miaodao islands, also known as Long Island. 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.