How is a mirage formed and why is the air density different?

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.

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.