What is a mirage?

[hìI shìshèn lóu]？

Mirage, referred to as mirage for short, is a natural phenomenon formed by refraction and total reflection of light, and a virtual image formed by refraction of light reflected by objects on the earth.

Mirages often appear in the sea and desert. Mirage is the result of light refraction along a straight line in gas layers with different densities. There are many kinds of mirage: according to its position relative to the original, it can be divided into three types: upper, lower and side; 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.

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.

As far as the formation of taking down the slug is concerned. 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 the sunlight enters the low-density air layer from the high-density air layer, the angle of the light changes, and after the refraction of the 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 the cold air on the sea surface and the warm air at high altitude is different, the light is refracted.

cause

principle

Refraction of light: When light is obliquely incident from one medium to another, its propagation direction changes, so that light is deflected at the junction of different media. Understanding: the refraction of light, like the reflection of light, occurs at the junction of two media, except that the reflected light returns to the original medium and the refracted light enters another medium. Because the propagation speed of light in two different substances is different, the propagation direction changes at the junction of two media, which is the refraction of light.

kind

Phantom diagram

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. The next mirage will appear in the desert in hot season or in the sea area 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.

When the atmospheric density in the horizontal direction is so different that the atmospheric refractive index in the horizontal direction is so different, a horizontal mirage may appear.

In a word, mirage is a very interesting phenomenon of refraction and reflection.

discuss

In summer, the temperature of sea water is relatively low during the day, especially on the sea surface where cold water flows. The water temperature is lower, and the lower air is colder than the upper air due to the influence of the water temperature, so there is an abnormal phenomenon of cooling and heating (under normal circumstances, it is cooling and heating, with an average increase of 100 m and a temperature drop of about 0.6℃). Because of the high air pressure and high density, the density of the lower air layer is particularly high, and the temperature is lower than that of the upper air layer, so the difference between the density under the air layer and the density 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.

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.

main feature

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.

Internal essence

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 and Emperor Wudi led people to visit Penglai Wonderland and 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.