mirage(hǎi shì shèn lóu)

mirage

This entry is reviewed by the project of "science popularization China" science encyclopedia entry compilation and application.

Mirage, also known as mirage, is a natural phenomenon formed by the refraction and total reflection of light. It is a virtual image formed by the atmospheric refraction of the light reflected by objects on the earth. Its essence is an optical phenomenon.

The appearance of mirage is closely related to geographical location, geophysical conditions and meteorological characteristics of those places at a specific time. It is characterized by repeated appearance at the same place and the same time.

Basic introduction

In calm sea surface, river surface, lake surface, snow plain, desert or Gobi and other places, occasionally in the air or "underground" appear tall buildings, city outline, trees and other illusions, which is called mirage. This kind of mirage often appears on the sea surface of Penglai, Shandong Province, China. The ancients attributed it to the mirage of Jiaolong, which breathed out air to form the outline of the tower and city, hence the name. Mirage is an optical phenomenon. The rise of hot air on the sandy or rocky surface causes the refraction of light, thus creating a mirage. Mirages can occur in desert areas about 6 miles (9.6 kilometers) from the coastline, making objects 1 mile (1.6 kilometers) away or further appear to be moving. It makes land navigation very difficult, because in a mirage environment, natural features become blurred. Mirage can make it difficult for a person to distinguish distant objects, and it can also blur the outline of the distant vision. You feel as if you are surrounded by a piece of water, and the higher part of that area looks like an "island" in the water. Mirage will also make it very difficult for you to identify the target, estimate the range and find people. However, if you go to a higher place, 10 feet (about 3 meters) (3 and 1 / 3 meters) above the desert ground, you can avoid the hot air close to the surface and overcome the mirage. In short, as long as the height of the wait-and-see is slightly adjusted, the mirage phenomenon will disappear, or its appearance and height will change. Mirage is often produced in the sea and desert. Mirage is the result of refraction of light in the air layer with different density along the straight line. There are many kinds of Mirage: according to its position relative to the original object, it can be divided into upper mirage, lower mirage and side mirage; according to its symmetrical relationship with the original object, it can be divided into positive mirage, side mirage, Shun mirage and anti mirage; according to its color, it can be divided into color mirage and non color mirage, etc.

Inner essence

Mirage has been concerned by the world since ancient times. In Western mythology, mirage is depicted as the incarnation of the devil, which is the omen of death and misfortune. In ancient China, mirage was regarded as a fairyland. Emperor Qinshihuang and Emperor Hanwu once led people to Penglai to visit the fairyland, and sent people to Penglai many times to seek a panacea. Modern science has made a correct explanation for most mirages. It is believed that mirage is a virtual image formed by the refraction of the light reflected by objects on the earth through the atmosphere. The so-called mirage is an optical mirage.

Formation conditions

Mirage is closely related to 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 mirages. Let's take the formation of mirage. In summer, when the sun is burning in the desert, the sand is scorching. Because the specific heat of the sand is small, the temperature rises very fast. The temperature of the lower air near the sand rises very high, but the temperature of the upper air is still very low. This forms an abnormal distribution of temperature. Due to the expansion and contraction of heat, the density of the lower hot air near the sand is small, while the density of the upper cold air is large, so the refractive index of the air is high It's small in the lower and big in the upper. When the light reflected from a higher object in the distance is refracted continuously from the denser air in the upper layer to the sparser air in the lower layer, the incident angle increases gradually, and the total reflection occurs when the incident angle is equal to the critical angle. At this time, if people look against the reflected light, they will see mirage. Because of the deep color of the road surface and the strong absorption capacity in the hot sun in summer, the asphalt road will also form the distribution characteristics of cold and high density air in the upper layer and low density air in the lower layer, so it will also form mirage. the "mirage" in the desert is the refraction of sunlight when it encounters air of 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 the air, when there is no wind, the vertical temperature difference over the desert is very significant. When the sunlight enters the air layer with low density from the air layer with high density, the angle of the light changes. Through the refraction of the light, the oasis in the distance will appear in front of people's eyes. In the sea or river, sometimes this "mirage" phenomenon will appear. Mirage is a kind of optical illusion, which is a virtual image formed by the refraction of the light reflected by objects on the earth through the atmosphere. Mirage is called mirage for short. According to the principle of physics, mirage is due to different air layers have different densities, and light has different refractive index in the air with different densities. That is because of the different density between the cold air on the sea and the warm air in the upper air, which refracts the light.

Cause of formation

Mirage is a kind of mirage formed by the refraction of distant objects, which is caused by the temperature gradient of the ground caused by the sun. Due to different density, light will refract at the boundary of temperature gradient. Our brain thinks that light always travels in a straight line, but when it passes through the low temperature and high density atmosphere below, it will refract downward, so the distant objects in our brain will be higher than they are. mirages often occur on the sea, because the air humidity in a certain area of the sea is relatively high, and the thickness is also relatively large, so the large area of water vapor forms a huge lens system under the motion. When the temperature near the ground changes dramatically, it will cause a huge difference in atmospheric density. When the light propagates, the distant scenery will have abnormal refraction and total reflection, resulting in mirage. In addition, the occurrence of mirage is also related to the refractive index of the air layer at different heights, and the moisture content is an important meteorological factor that directly affects the refractive index. At the same time, it also has a certain relationship with the sudden change of temperature and relative humidity. The air near the sea surface has a higher refractive index because of the low temperature of the sea water and the moisture of the water vapor, while the air above the sea surface has a higher temperature because of the influence of sunlight. The refractive index of the air layer over the sea surface decreases gradually with the height from bottom to top. Due to the discontinuity of meteorological elements, the refractive index of light in the air also shows discontinuity. When light passes through the top of the air layer with large refractive index, it is easy to form total reflection under sunny, windless or breezy weather conditions, resulting in the mirage spectacle.

Formation principle

Refraction of light: when light slants from one medium into another medium, its propagation direction changes, so that light deflects at the junction of different media. Understanding: the refraction of light and the reflection of light both occur at the junction of the two media, but the reflected light returns to the original media, and the refracted light enters into another media. Because the speed of light propagation is different in two different materials, the direction of light propagation changes at the junction of the two media, which is the refraction of light. experiment someone has used a device to project light from water onto the interface between water and air. You can see that the light at this interface is divided into two parts: one is reflected into water, and the other is refracted into air. If you turn the mirror in the water to make the light to the interface more inclined, the refraction of light in the air will be more severe.

simulation experiment

In an airless room, a piece of smooth iron sheet with a length of 1.5 meters and a width of 20 cm is horizontally placed on several small pillars made of iron pipes (or wooden sticks), and a thin layer of sand is sprinkled on the iron sheet to make a desert like surface. Cut trees and camels with dark colored paper and stick them on a piece of ground glass (milky white glass). Put the glass plate on one end of the iron sheet, perpendicular to the iron sheet, so that the trees and camels can be exposed on the sand. Under the back of the glass plate, shine up with a flashlight. At the other end of the iron plate, it looks like the bright sky behind the trees and camels. Then, three small briquette furnaces are put under the iron sheet for heating (or a long carbon basin is used, when conditions permit, the long electric furnace is the most ideal). When heating, pay attention to the uniform heating of the iron sheet, especially near two-thirds of the ground glass end. in this way, after heating for a certain period of time, when the hand is close to the sand surface and feels very hot, it begins to observe along the thin iron sheet towards the ground glass. You can see the reflection of trees and camels under the sand, just like the reflection of a lake beside trees and camels. This phenomenon is "mirage". In hot summer, sometimes you can see the reflection of houses and trees on the asphalt road. This is actually a mirage phenomenon.

characteristic

Mirage has two characteristics: first, it appears repeatedly in the same place, for example, mirage often appears over Alaska in the United States; second, it appears at the same time, for example, mirage in Penglai mostly appears in May and June every year, mirage near zimlensk in Russia often appears in spring, while mirage in Alaska usually appears in 20 days after June 20.

category

Mirage is an abnormal refraction phenomenon. It is the result of refraction of light propagating in the atmosphere with different vertical density. It is often divided into upper, lower and side mirages.

Chinese PinYin : hǎi shì shèn lóu