In a calm atmosphere the situation is observed. Text assignments (GY in physics). Planets and stars: what's the difference

There is a lot of interesting things in the world. The twinkling of stars is one of the most amazing phenomena. How many different beliefs are associated with this phenomenon! The unknown always frightens and attracts at the same time. What is the nature of this phenomenon?

Influence of the atmosphere

Astronomers have made an interesting discovery: the twinkling of stars has nothing to do with their changes. Then why do stars twinkle in the night sky? It's all about the atmospheric movement of cold and hot air flows. Where warm layers pass over cold ones, air vortices are formed. Under the influence of these vortices, the rays of light are distorted. This is how light rays bend, changing the apparent position of stars.

An interesting fact is that the stars do not twinkle at all. This vision is created on earth. Observers' eyes perceive light coming from a star after it passes through the atmosphere. Therefore, to the question of why stars twinkle, we can answer that stars do not twinkle, but the phenomenon that we observe on earth is a distortion of light that has passed from a star through the atmospheric layers of air. If such air movements did not occur, then flickering would not be observed, even from the most distant star in space.

Scientific explanation

If we expand on the question of why stars twinkle in more detail, it is worth noting that this process is observed when light from a star moves from a denser atmospheric layer to a less dense one. In addition, as mentioned above, these layers are constantly moving relative to each other. From the laws of physics we know that warm air rises, and cold air, on the contrary, sinks. It is when light passes this layer boundary that we observe flickering.

Passing through layers of air of different density, the light of the stars begins to flicker, and their outlines blur and the image increases. At the same time, the radiation intensity and, accordingly, brightness also change. Thus, by studying and observing the processes described above, scientists understood why stars twinkle, and their flickering varies in intensity. In science, this change in light intensity is called scintillation.

Planets and stars: what's the difference?

Another interesting fact is that not every luminous cosmic object produces light emanating from the phenomenon of scintillation. Let's take the planets. They also reflect sunlight, but do not flicker. It is by the nature of radiation that a planet is distinguished from a star. Yes, the light of a star flickers, but the light of a planet does not.

Since ancient times, humanity has learned to navigate in space using the stars. In those days when precision instruments were not invented, the sky helped to find the right path. And today this knowledge has not lost its significance. Astronomy as a science began in the 16th century, when the telescope was first invented. That’s when they began to closely observe the light of the stars and study the laws by which they twinkle. Word astronomy translated from Greek it is “the law of the stars.”

Star Science

Astronomy studies the Universe and celestial bodies, their movement, location, structure and origin. Thanks to the development of science, astronomers have explained how a twinkling star in the sky differs from a planet, how the development of celestial bodies, their systems, and satellites occurs. This science has looked far beyond the boundaries of the solar system. Pulsars, quasars, nebulae, asteroids, galaxies, black holes, interstellar and interplanetary matter, comets, meteorites and everything related to outer space are studied by the science of astronomy.

The intensity and color of the twinkling starlight is also influenced by the altitude of the atmosphere and proximity to the horizon. It is easy to notice that stars located close to it shine brighter and shimmer in different colors. This sight becomes especially beautiful on frosty nights or immediately after rain. At these moments the sky is cloudless, which contributes to a brighter flicker. Sirius has a special radiance.

Atmosphere and starlight

If you want to observe the twinkling of stars, you should understand that with a calm atmosphere at the zenith, this is only possible occasionally. The brightness of the light flux is constantly changing. This is again due to the deflection of light rays, which are unevenly concentrated above the earth's surface. The wind also influences the starscape. In this case, the observer of the star panorama constantly finds himself alternately in a darkened or illuminated area.

When observing stars located at an altitude of more than 50°, the color change will not be noticeable. But stars that are below 35° will twinkle and change color quite often. Very intense flickering indicates atmospheric heterogeneity, which is directly related to meteorology. While observing stellar twinkling, it was noticed that it tends to intensify at low atmospheric pressure and temperature. An increase in flicker can also be noticed with increasing humidity. However, it is impossible to predict the weather using scintillation. The state of the atmosphere depends on a large number of different factors, which does not allow us to draw conclusions about the weather only from stellar twinkling. Of course, some things work, but this phenomenon still has its own ambiguities and mysteries.

Have you ever wondered why the stars are not visible in the sky during the daytime? After all, the air is as transparent during the day as it is at night. The whole point here is that during the daytime the atmosphere scatters sunlight.

Imagine that you are in a well-lit room in the evening. Through the window glass, bright lights located outside are visible quite clearly. But dimly lit objects are almost impossible to see. However, as soon as you turn off the light in the room, the glass ceases to serve as an obstacle to our vision.

Something similar happens when observing the sky: during the day, the atmosphere above us is brightly illuminated and the Sun is visible through it, but the weak light of distant stars cannot penetrate. But after the Sun sinks below the horizon and the sunlight (and with it the light scattered by the air) “turns off,” the atmosphere becomes “transparent” and the stars can be observed.

It's a different matter in space. As the spacecraft rises to altitude, dense layers of the atmosphere remain below and the sky gradually darkens.

At an altitude of about 200-300 km, where manned spacecraft usually fly, the sky is completely black. It is always black, even if the Sun is currently on the visible part of it.

“The sky is completely black. The stars in this sky look somewhat brighter and are more clearly visible against the background of the black sky,” this is how the first cosmonaut Yu. A. Gagarin described his space impressions.

But still, even from the spacecraft on the day side of the sky, not all the stars are visible, but only the brightest. The eye is disturbed by the blinding light of the Sun and the light of the Earth.

If we look at the sky from Earth, we will clearly see that all the stars are twinkling. They seem to fade, then flare up, shimmering with different colors. And the lower the star is located above the horizon, the stronger the flickering.

The twinkling of stars is also explained by the presence of an atmosphere. Before reaching our eyes, the light emitted by a star passes through the atmosphere. In the atmosphere there are always masses of warmer and colder air. Its Density depends on the temperature of the air in a particular area. Passing from one area to another, light rays experience refraction. The direction of their propagation changes. Due to this, in some places above the earth's surface they are concentrated, in others they are relatively rare. As a result of the constant movement of air masses, these zones are constantly shifting, and the observer sees either an increase or decrease in the brightness of the stars. But since different colored rays are not refracted equally, the moments of intensification and weakening of different colors do not occur simultaneously.

In addition, other, more complex optical effects can play a certain role in the twinkling of stars.

The presence of warm and cold layers of air and intense movements of air masses also affect the quality of telescopic images.

Where are the best conditions for astronomical observations: in the mountains or on the plains, on the seashore or inland, in the forest or in the desert? And in general, what is better for astronomers - ten cloudless nights over the course of a month or just one clear night, but one when the air is perfectly clear and calm?

This is only a small part of the issues that have to be resolved when choosing a location for the construction of observatories and the installation of large telescopes. A special field of science deals with such problems - astro-climatology.

Of course, the best conditions for astronomical observations are outside the dense layers of the atmosphere, in space. By the way, the stars here do not twinkle, but burn with a cold, calm light.

Familiar constellations look exactly the same in space as they do on Earth. The stars are at enormous distances from us, and moving away from the earth's surface by a few hundred kilometers cannot change anything in their apparent relative position. Even when observed from Pluto, the outlines of the constellations would be exactly the same.

During one orbit from a spacecraft moving in low-Earth orbit, in principle, you can see all the constellations of the earth's sky. Observing stars from space is of dual interest: astronomical and navigational. In particular, it is very important to observe starlight unmodified by the atmosphere.

Navigation by the stars is no less important in space. By observing pre-selected “reference” stars, you can not only orient the ship, but also determine its position in space.

For a long time, astronomers have dreamed of future observatories on the surface of the Moon. It seemed that the complete absence of an atmosphere should create ideal conditions on the Earth’s natural satellite for astronomical observations both during the lunar night and during the lunar day.

Passing through the earth's atmosphere, light rays change their straight direction. Due to the increase in atmospheric density, the refraction of light rays increases as they approach the Earth's surface. As a result, the observer sees the celestial bodies as if raised above the horizon by an angle called astronomical refraction.

Refraction is one of the main sources of both systematic and random observation errors. In 1906 Newcomb wrote that there is no branch of practical astronomy that has been written about so much as refraction, and which would be in such an unsatisfactory state. Until the mid-20th century, astronomers reduced their observations using refraction tables compiled in the 19th century. The main drawback of all old theories was an inaccurate understanding of the structure of the earth's atmosphere.

Let us take the surface of the Earth AB as a sphere of radius OA=R, and imagine the Earth’s atmosphere in the form of layers concentric with it aw, a 1 in 1, and 2 in 2...with densities increasing as the layers approach the earth's surface (Fig. 2.7). Then a ray SA from some very distant body, refracted in the atmosphere, will arrive at point A in the direction S¢A, deviating from its initial position SA or from the direction S²A parallel to it by a certain angle S¢AS²= r, called astronomical refraction. All elements of the curved ray SA and its final apparent direction AS¢ will lie in the same vertical plane ZAOS. Consequently, astronomical refraction only increases the true direction to the luminary in the vertical plane passing through it.

The angular elevation of a star above the horizon in astronomy is called the height of the star. Angle S¢AH = h¢ will be the apparent height of the star, and the angle S²AH = h = h¢ - r is its true height. Corner z is the true zenith distance of the luminary, and z¢ is its visible value.

The amount of refraction depends on many factors and can change in every place on Earth, even within a day. For average conditions, an approximate refraction formula was obtained:

Dh=-0.9666ctg h¢. (2.1)

The coefficient 0.9666 corresponds to the density of the atmosphere at a temperature of +10°C and a pressure of 760 mm Hg. If the characteristics of the atmosphere are different, then the correction for refraction, calculated according to formula (2.1), must be corrected by corrections for temperature and pressure.

Fig. 2.7. Astronomical refraction

To take into account astronomical refraction in zenithal methods of astronomical determinations, temperature and air pressure are measured during observation of the zenith distances of luminaries. In precise methods of astronomical determinations, the zenith distances of luminaries are measured in the range from 10° to 60°. The upper limit is due to instrumental errors, the lower limit is due to errors in the refraction tables.

The zenith distance of the luminary, corrected by the refraction correction, is calculated by the formula:

Average (normal at a temperature of +10°C and a pressure of 760 mm Hg.) refraction, calculated by z¢;

A coefficient that takes into account air temperature, calculated from the temperature value;

B– coefficient taking into account air pressure.

Many scientists studied the theory of refraction. Initially, the initial assumption was that the density of various layers of the atmosphere decreases with increasing height of these layers in an arithmetic progression (Bouguer). But this assumption was soon recognized as unsatisfactory in all respects, since it led to too small a value of refraction and to a too rapid decrease in temperature with height above the Earth's surface.

Newton hypothesized that the density of the atmosphere decreases with height according to the law of geometric progression. And this hypothesis turned out to be unsatisfactory. According to this hypothesis, it turned out that the temperature in all layers of the atmosphere should remain constant and equal to the temperature on the surface of the Earth.

The most ingenious was Laplace's hypothesis, intermediate between the two above. The refraction tables that were published annually in the French astronomical calendar were based on this Laplace hypothesis.

The Earth's atmosphere with its instability (turbulence, refractive variations) places a limit on the accuracy of astronomical observations from Earth.

When choosing a site for installing large astronomical instruments, the astroclimate of the area is first comprehensively studied, which is understood as a set of factors that distort the shape of the wave front of radiation from celestial objects passing through the atmosphere. If the wave front reaches the device undistorted, then the device in this case can operate with maximum efficiency (with a resolution approaching the theoretical one).

As it turned out, the quality of the telescopic image is reduced mainly due to interference introduced by the ground layer of the atmosphere. The earth, due to its own thermal radiation at night, cools significantly and cools the adjacent layer of air. A change in air temperature by 1°C changes its refractive index by 10 -6. On isolated mountain peaks, the thickness of the ground layer of air with a significant temperature difference (gradient) can reach several tens of meters. In valleys and flat areas at night, this layer is much thicker and can be hundreds of meters. This explains the choice of sites for astronomical observatories on the spurs of ridges and on isolated peaks, from where denser cold air can flow into the valleys. The height of the telescope tower is chosen such that the instrument is located above the main region of temperature inhomogeneities.

An important factor in astroclimate is the wind in the surface layer of the atmosphere. By mixing layers of cold and warm air, it causes the appearance of density inhomogeneities in the air column above the device. Inhomogeneities whose dimensions are smaller than the diameter of the telescope lead to defocusing of the image. Larger density fluctuations (several meters or larger) do not cause sharp distortions of the wave front and lead mainly to displacement rather than defocusing of the image.

In the upper layers of the atmosphere (at the tropopause), fluctuations in the density and refractive index of air are also observed. But disturbances in the tropopause do not noticeably affect the quality of images produced by optical instruments, since temperature gradients there are much smaller than in the surface layer. These layers do not cause trembling, but the twinkling of stars.

In astroclimatic studies, a connection is established between the number of clear days recorded by the weather service and the number of nights suitable for astronomical observations. The most advantageous areas, according to astroclimatic analysis of the territory of the former USSR, are some mountainous regions of the Central Asian states.

Terrestrial refraction

Rays from ground objects, if they travel a long enough path in the atmosphere, also experience refraction. The trajectory of rays is bent under the influence of refraction, and we see them in the wrong places or in the wrong direction where they actually are. Under certain conditions, as a result of terrestrial refraction, mirages appear - false images of distant objects.

The angle of terrestrial refraction a is the angle between the direction to the apparent and actual position of the observed object (Fig. 2.8). The value of the angle a depends on the distance to the observed object and on the vertical temperature gradient in the surface layer of the atmosphere, in which the propagation of rays from ground objects occurs.

Fig.2.8. Manifestation of terrestrial refraction during sighting:

a) – from bottom to top, b) – from top to bottom, a – angle of terrestrial refraction

The geodetic (geometric) visibility range is associated with terrestrial refraction (Fig. 2.9). Let us assume that the observer is at point A at a certain height hH above the earth's surface and observes the horizon in the direction of point B. The NAN plane is a horizontal plane passing through point A perpendicular to the radius of the globe, called the plane of the mathematical horizon. If rays of light propagated rectilinearly in the atmosphere, then the farthest point on Earth that an observer from point A could see would be point B. The distance to this point (tangent AB to the globe) is the geodetic (or geometric) visibility range D 0 . A circular line on the earth's surface explosive is the geodetic (or geometric) horizon of the observer. The value of D 0 is determined only by geometric parameters: the radius of the Earth R and the height h H of the observer and is equal to D o ≈ √ 2Rh H = 3.57√ h H, which follows from Fig. 2.9.

Fig.2.9. Terrestrial refraction: mathematical (NN) and geodetic (BB) horizons, geodetic visibility range (AB=D 0)

If an observer observes an object located at a height h above the Earth's surface, then the geodetic range will be the distance AC = 3.57(√ h H + √ h pr). These statements would be true if light traveled in a straight line through the atmosphere. But that's not true. With a normal distribution of temperature and air density in the ground layer, the curved line depicting the trajectory of the light beam faces the Earth with its concave side. Therefore, the farthest point that an observer from A will see will not be B, but B¢. The geodetic visibility range AB¢, taking into account refraction, will be on average 6-7% greater and instead of the coefficient of 3.57 in the formulas there will be a coefficient of 3.82. Geodetic range is calculated using the formulas

, h - in m, D - in km, R - 6378 km

Where h n and h pr – in meters, D – in kilometers.

For a person of average height, the horizon distance on Earth is about 5 km. For cosmonauts V.A. Shatalov and A.S. Eliseev, who flew on the Soyuz-8 spacecraft, the horizon range at perigee (altitude 205 km) was 1730 km, and at apogee (altitude 223 km) - 1800 km.

For radio waves, refraction is almost independent of wavelength, but in addition to temperature and pressure, it also depends on the water vapor content in the air. Under the same conditions of temperature and pressure changes, radio waves are refracted more strongly than light ones, especially with high humidity.

Therefore, in the formulas for determining the range of the horizon or detecting an object by a radar beam in front of the root there will be a coefficient of 4.08. Consequently, the horizon of the radar system is approximately 11% further away.

Radio waves are well reflected from the earth's surface and from the lower boundary of the inversion or layer of low humidity. In such a unique waveguide formed by the earth's surface and the base of the inversion, radio waves can propagate over very long distances. These features of radio wave propagation are successfully used in radar.

The air temperature in the ground layer, especially in its lower part, does not always fall with height. It can decrease at different rates, it may not change with height (isothermia) and it can increase with height (inversion). Depending on the magnitude and sign of the temperature gradient, refraction can have different effects on the range of the visible horizon.

The vertical temperature gradient in a homogeneous atmosphere in which the air density does not change with height, g 0 = 3.42°C/100m. Let's consider what the ray trajectory will be AB at different temperature gradients at the Earth's surface.

Let , i.e. air temperature decreases with altitude. Under this condition, the refractive index also decreases with height. The trajectory of the light beam in this case will be facing the earth's surface with its concave side (in Fig. 2.9 the trajectory AB¢). This refraction is called positive. Farthest point IN¢ the observer will see in the direction of the last tangent to the ray path. This tangent, i.e. the horizon visible due to refraction is equal to the mathematical horizon NAS angle D, less than angle d. Corner d is the angle between the mathematical and geometric horizon without refraction. Thus, the visible horizon has risen by an angle ( d- D) and expanded because D > D0.

Now let's imagine that g gradually decreases, i.e. Temperature decreases more and more slowly with altitude. There will come a moment when the temperature gradient becomes zero (isothermia), and then the temperature gradient becomes negative. The temperature no longer decreases, but increases with altitude, i.e. temperature inversion is observed. As the temperature gradient decreases and passes through zero, the visible horizon will rise higher and higher and a moment will come when D becomes equal to zero. The visible geodetic horizon will rise to the mathematical one. The earth's surface seemed to straighten out and become flat. The geodetic visibility range is infinitely large. The radius of curvature of the beam became equal to the radius of the globe.

With an even stronger temperature inversion, D becomes negative. The visible horizon has risen above the mathematical one. It will seem to the observer at point A that he is at the bottom of a huge basin. Because of the horizon, objects located far beyond the geodetic horizon rise and become visible (as if floating in the air) (Fig. 2.10).

Such phenomena can be observed in polar countries. So, from the Canadian coast of America through Smith Strait you can sometimes see the coast of Greenland with all the buildings on it. The distance to the Greenland coast is about 70 km, while the geodetic visibility range is no more than 20 km. Another example. From Hastings, on the English side of the Pas-de-Calais Strait, I could see the French coast, lying across the Strait at a distance of about 75 km.

Fig.2.10. The phenomenon of unusual refraction in polar countries

Now let's assume that g=g 0, therefore, the air density does not change with height (homogeneous atmosphere), there is no refraction and D=D 0 .

At g > g 0 the refractive index and air density increase with altitude. In this case, the trajectory of light rays faces the earth's surface with its convex side. This refraction is called negative. The last point on Earth that an observer at A will see will be B². The visible horizon AB² narrowed and dropped to an angle (D - d).

From what has been discussed, we can formulate the following rule: if along the propagation of a light beam in the atmosphere the air density (and, therefore, the refractive index) changes, then the light beam will bend so that its trajectory is always convex in the direction of decreasing the density (and refractive index) of the air .

Refraction and mirages

The word mirage is of French origin and has two meanings: “reflection” and “deceptive vision.” Both meanings of this word well reflect the essence of the phenomenon. A mirage is an image of an object that actually exists on Earth, often enlarged and greatly distorted. There are several types of mirages depending on where the image is located in relation to the object: upper, lower, lateral and complex. The most commonly observed are superior and inferior mirages, which occur when there is an unusual distribution of density (and, therefore, refractive index) in height, when at a certain height or near the surface of the Earth there is a relatively thin layer of very warm air (with a low refractive index), in which Rays coming from ground objects experience total internal reflection. This occurs when rays fall on this layer at an angle greater than the angle of total internal reflection. This warmer layer of air plays the role of an air mirror, reflecting the rays falling into it.

Superior mirages (Fig. 2.11) occur in the presence of strong temperature inversions, when air density and refractive index rapidly decrease with height. In superior mirages, the image is located above the object.

Fig.2.11. Superior Mirage

The trajectories of light rays are shown in Figure (2.11). Let us assume that the earth's surface is flat and layers of equal density are located parallel to it. Since density decreases with height, then . The warm layer, which acts as a mirror, lies at a height. In this layer, when the angle of incidence of the rays becomes equal to the refractive index (), the rays rotate back to the earth's surface. The observer can simultaneously see the object itself (if it is not beyond the horizon) and one or more images above it - upright and inverted.

Fig.2.12. Complex superior mirage

In Fig. Figure 2.12 shows a diagram of the occurrence of a complex upper mirage. The object itself is visible ab, above him there is a direct image of him a¢b¢, inverted in²b² and again direct a²¢b²¢. Such a mirage can occur if the air density decreases with altitude, first slowly, then quickly, and again slowly. The image turns out upside down if the rays coming from the extreme points of the object intersect. If an object is far away (beyond the horizon), then the object itself may not be visible, but its images, raised high in the air, are visible from great distances.

The city of Lomonosov is located on the shores of the Gulf of Finland, 40 km from St. Petersburg. Usually from Lomonosov St. Petersburg is not visible at all or is visible very poorly. Sometimes St. Petersburg is visible “at a glance.” This is one example of superior mirages.

Apparently, the number of upper mirages should include at least part of the so-called ghostly Lands, which were searched for decades in the Arctic and were never found. They searched for Sannikov Land for a particularly long time.

Yakov Sannikov was a hunter and was involved in the fur trade. In 1811 He set off on dogs across the ice to the group of New Siberian Islands and from the northern tip of Kotelny Island saw an unknown island in the ocean. He was unable to reach it, but reported the discovery of a new island to the government. In August 1886 E.V. Tol, during his expedition to the New Siberian Islands, also saw Sannikov Island and wrote in his diary: “The horizon is completely clear. In the direction to the northeast, 14-18 degrees, the contours of four mesas were clearly visible, which connected to the low-lying land in the east. Thus, Sannikov’s message was completely confirmed. We have the right, therefore, to draw a dotted line in the appropriate place on the map and write on it: “Sannikov Land.”

Tol gave 16 years of his life to the search for Sannikov Land. He organized and conducted three expeditions to the New Siberian Islands area. During the last expedition on the schooner “Zarya” (1900-1902), Tolya’s expedition died without finding Sannikov Land. No one saw Sannikov Land again. Perhaps it was a mirage that appears in the same place at certain times of the year. Both Sannikov and Tol saw a mirage of the same island located in this direction, only much further in the ocean. Perhaps it was one of the De Long Islands. Perhaps it was a huge iceberg - an entire ice island. Such ice mountains, with an area of ​​up to 100 km2, travel across the ocean for several decades.

The mirage did not always deceive people. English polar explorer Robert Scott in 1902. in Antarctica I saw mountains as if hanging in the air. Scott suggested that there was a mountain range further beyond the horizon. And, indeed, the mountain range was discovered later by the Norwegian polar explorer Raoul Amundsen exactly where Scott expected it to be located.

Fig.2.13. Inferior Mirage

Inferior mirages (Fig. 2.13) occur with a very rapid decrease in temperature with height, i.e. at very large temperature gradients. The role of an air mirror is played by the thin surface warmest layer of air. A mirage is called an inferior mirage because the image of an object is placed under the object. In lower mirages, it seems as if there is a surface of water under the object and all objects are reflected in it.

In calm water, all objects standing on the shore are clearly reflected. Reflection in a thin layer of air heated from the earth's surface is completely similar to reflection in water, only the role of a mirror is played by the air itself. The air condition in which inferior mirages occur is extremely unstable. After all, below, near the ground, lies highly heated, and therefore lighter, air, and above it lies colder and heavier air. Jets of hot air rising from the ground penetrate layers of cold air. Due to this, the mirage changes before our eyes, the surface of the “water” seems to be agitated. A small gust of wind or a shock is enough and a collapse will occur, i.e. turning over air layers. Heavy air will rush down, destroying the air mirror, and the mirage will disappear. Favorable conditions for the occurrence of inferior mirages are a homogeneous, flat underlying surface of the Earth, which occurs in steppes and deserts, and sunny, windless weather.

If a mirage is an image of a really existing object, then the question arises: what kind of water surface do travelers in the desert see? After all, there is no water in the desert. The fact is that the apparent water surface or lake visible in a mirage is in fact an image not of the water surface, but of the sky. Parts of the sky are reflected in the air mirror and create the complete illusion of a shiny water surface. Such a mirage can be seen not only in the desert or steppe. They even appear in St. Petersburg and its environs on sunny days over asphalt roads or a flat sandy beach.

Fig.2.14. Side mirage

Side mirages occur in cases where layers of air of the same density are located in the atmosphere not horizontally, as usual, but obliquely and even vertically (Fig. 2.14). Such conditions are created in the summer, in the morning shortly after sunrise, on the rocky shores of the sea or lake, when the shore is already illuminated by the Sun, and the surface of the water and the air above it are still cold. Lateral mirages have been repeatedly observed on Lake Geneva. A side mirage can appear near a stone wall of a house heated by the Sun, and even on the side of a heated stove.

Complex types of mirages, or Fata Morgana, occur when there are simultaneously conditions for the appearance of both an upper and lower mirage, for example, during a significant temperature inversion at a certain altitude above a relatively warm sea. Air density first increases with height (air temperature decreases), and then also quickly decreases (air temperature rises). With such a distribution of air density, the state of the atmosphere is very unstable and subject to sudden changes. Therefore, the appearance of the mirage changes before our eyes. The most ordinary rocks and houses, due to repeated distortions and magnification, turn into the wonderful castles of the fairy Morgana before our eyes. Fata Morgana is observed off the coast of Italy and Sicily. But it can also occur at high latitudes. This is how the famous Siberian explorer F.P. Wrangel described the Fata Morgana he saw in Nizhnekolymsk: “The action of horizontal refraction produced a kind of Fata Morgana. The mountains lying to the south seemed to us in various distorted forms and hanging in the air. The distant mountains seemed to have their peaks overturned. The river narrowed to the point that the opposite bank seemed to be almost at our huts.”

Job source: Solution 4555. OGE 2017 Physics, E.E. Kamzeeva. 30 options.

Task 20. In the text, refraction refers to the phenomenon

1) changes in the direction of propagation of the light beam due to reflection at the boundary of the atmosphere

2) changes in the direction of propagation of a light beam due to refraction in the Earth’s atmosphere

3) absorption of light as it propagates in the Earth’s atmosphere

4) the light beam bends around obstacles and thereby deviates from straight-line propagation

Solution.

Before a ray of light from a distant space object (such as a star) can enter the eye of an observer, it must pass through the earth's atmosphere. In this case, the light beam undergoes the processes of refraction, absorption and scattering.

Refraction of light in the atmosphere is an optical phenomenon caused by the refraction of light rays in the atmosphere and manifested in the apparent displacement of distant objects (for example, stars observed in the sky). As the light ray from a celestial body approaches the surface of the Earth, the density of the atmosphere increases (Fig. 1), and the rays are refracted more and more. The process of propagation of a light beam through the earth's atmosphere can be simulated using a stack of transparent plates, the optical density of which changes as the beam propagates.

Due to refraction, the observer sees objects not in the direction of their actual position, but along the tangent to the beam path at the observation point (Fig. 3). The angle between the true and apparent directions of an object is called the angle of refraction. Stars near the horizon, whose light must pass through the greatest thickness of the atmosphere, are most susceptible to atmospheric refraction (the refraction angle is about 1/6 of an angular degree).