"Mountain City", like sea city, is a natural scene formed by refraction and reflection. It is beautiful and strange, but it rarely appears once in several years. Only a few lucky people can see it by chance, while most people can only look forward to it and sigh.
When the light is of the same density, When proceeding in a uniform medium, the speed of light remains unchanged and it advances in a straight direction. However, when the light moves obliquely from this medium into another medium with different densities, the speed of light will change and the direction of progression will change. It also bends, a phenomenon 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 increasing 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.
However, when the air temperature changes abnormally in the vertical direction, which will lead to different refraction and total reflection than usual, this will produce a mirage phenomenon. Due to different specific conditions of abnormal air density, mirages appear in different forms.
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. It is cooler than the air in the upper layer. The abnormal phenomenon of cold rising to warm (normal conditions are warm at the bottom and cool at the top. On average, the temperature drops by about 0.5~0.6 every 100 meters in height). 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 under our eastern horizon, it will generally not be visible. If the difference between the dense air below and the thin air is too great at this time, the light from the ship will be gradually refracted from the dense air layer into the thin air layer, 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 Miaodao Islands in the Bohai Sea of ??my country. In summer, the sea water 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 ), the phantom of the Temple Island Islands can often be seen. Shen Kuo, who lived in the Song Dynasty, recorded this in his famous "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 with crowns, You can see it clearly."
This is the mirage he saw in Penglai. 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 throughout the 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 be seen on the river in the morning. For example, on August 2, 1934, it appeared on the river near Nantong. It was a cool day in the sky and the weather was extremely hot. In the afternoon, I suddenly found towers, city outlines, trees and houses reflected in the sky over 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 rocks 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 cause the upper and lower air to stir and mix, 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.