Ptolemy's experiments on the refraction of light. Astronomical refraction Thomson's experiments and the discovery of the electron
Greek astronomer Claudius Ptolemy (c. 130 AD) is the author of a remarkable book that served as the primary textbook on astronomy for nearly 15 centuries. However, in addition to the astronomical textbook, Ptolemy also wrote the book “Optics”, in which he outlined the theory of vision, the theory of flat and spherical mirrors and the study of the phenomenon of light refraction. Ptolemy encountered the phenomenon of light refraction while observing the stars. He noticed that a ray of light, moving from one medium to another, “breaks.” Therefore, a star ray, passing through the earth’s atmosphere, reaches the earth’s surface not in a straight line, but along a curved line, that is, refraction occurs. The curvature of the beam occurs due to the fact that the air density changes with altitude.
To study the law of refraction, Ptolemy conducted the following experiment. He took a circle and fixed the rulers l1 and l2 on the axis so that they could rotate freely around it (see figure). Ptolemy immersed this circle in water to the diameter AB and, turning the lower ruler, ensured that the rulers lay on the same straight line for the eye (if you look along the upper ruler). After this, he took the circle out of the water and compared the angles of incidence α and refraction β. It measured angles with an accuracy of 0.5°. The numbers obtained by Ptolemy are presented in the table.
Ptolemy did not find a “formula” for the relationship between these two series of numbers. However, if we determine the sines of these angles, it turns out that the ratio of the sines is expressed by almost the same number, even with such a rough measurement of angles, which Ptolemy resorted to.
Due to the refraction of light in a calm atmosphere, the apparent position of stars in the sky relative to the horizon
1) higher than actual position
2) below actual position
3) shifted to one side or another vertically relative to the actual position
4) matches the actual position
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In a calm atmosphere, the position of stars that are not perpendicular to the Earth’s surface at the point where the observer is located is observed. What is the apparent position of the stars - above or below their actual position relative to the horizon? Explain your answer.
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In the text, refraction refers to the phenomenon
1) changes in the direction of propagation of a 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 through the Earth's atmosphere
4) bending of a light beam around obstacles and thereby deviation from rectilinear propagation
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Which of the following conclusions contradicts Ptolemy's experiments?
1) the angle of refraction is less than the angle of incidence when the beam passes from air to water
2) As the angle of incidence increases, the angle of refraction increases linearly
3) the ratio of the sine of the angle of incidence to the sine of the angle of refraction does not change
4) the sine of the angle of refraction depends linearly on the sine of the angle of incidence
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Photoluminescence
Some substances themselves begin to glow when illuminated by electromagnetic radiation. This glow, or luminescence, has an important feature: the luminescent light has a different spectral composition than the light that caused the glow. Observations show that luminescence light has a longer wavelength than the exciting light. For example, if a beam of violet light is directed at a cone containing a fluorescein solution, the illuminated liquid begins to luminesce brightly with green-yellow light.
Some bodies retain the ability to glow for some time after their illumination has ceased. This afterglow can have different durations: from a fraction of a second to many hours. It is customary to call a glow that stops with illumination fluorescence, and a glow that has a noticeable duration is phosphorescence.
Phosphorescent crystalline powders are used to coat special screens that retain their glow for two to three minutes after illumination. Such screens also glow when exposed to X-rays.
Phosphorescent powders have found very important use in the manufacture of fluorescent lamps. In gas-discharge lamps filled with mercury vapor, ultraviolet radiation occurs when an electric current passes. Soviet physicist S.I. Vavilov proposed covering the inner surface of such lamps with a specially prepared phosphorescent composition, which produces visible light when irradiated with ultraviolet light. By selecting the composition of the phosphorescent substance, it is possible to obtain the spectral composition of the emitted light as close as possible to the spectral composition of daylight.
The phenomenon of luminescence is characterized by extremely high sensitivity: sometimes 10 – 10 g of a luminous substance, for example in a solution, is enough to detect this substance by its characteristic glow. This property is the basis of luminescent analysis, which makes it possible to detect negligible impurities and judge about contaminants or processes leading to changes in the original substance.
Human tissues contain a large number of diverse natural fluorophores, which have different fluorescence spectral regions. The figure shows the emission spectra of the main fluorophores of biological tissues and the scale of electromagnetic waves.
According to the data presented, pyroxidine glows
1) red light
2) yellow light
3) green light
4) purple light
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Two identical crystals, which have the property of phosphorescent in the yellow part of the spectrum, were preliminarily illuminated: the first with red rays, the second with blue rays. For which of the crystals can the afterglow be observed? Explain your answer.
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When examining food products, the luminescence method can be used to identify spoilage and falsification of products.
The table shows the luminescence indicators of fats.
The luminescence color of the butter changed from yellow-green to blue. This means that the butter may have been added
1) only creamy margarine
2) only “Extra” margarine
3) only vegetable lard
4) any of the following fats
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Albedo of the Earth
The temperature at the Earth's surface depends on the reflectivity of the planet - albedo. Surface albedo is the ratio of the energy flux of reflected solar rays to the energy flux of solar rays incident on the surface, expressed as a percentage or fraction of a unit. The Earth's albedo in the visible part of the spectrum is about 40%. In the absence of clouds it would be about 15%.
Albedo depends on many factors: the presence and condition of cloudiness, changes in glaciers, time of year, and, accordingly, precipitation.
In the 90s of the 20th century, the significant role of aerosols—“clouds” of tiny solid and liquid particles in the atmosphere—became obvious. When fuel is burned, gaseous sulfur and nitrogen oxides are released into the air; combining in the atmosphere with water droplets, they form sulfuric, nitric acids and ammonia, which then turn into sulfate and nitrate aerosols. Aerosols not only reflect sunlight, preventing it from reaching the Earth's surface. Aerosol particles serve as condensation nuclei for atmospheric moisture during cloud formation and thereby contribute to an increase in cloudiness. And this, in turn, reduces the flow of solar heat to the earth's surface.
Transparency to sunlight in the lower layers of the earth's atmosphere also depends on fires. Due to fires, dust and soot rise into the atmosphere, which cover the Earth with a dense screen and increase the albedo of the surface.
Which statements are true?
A. Aerosols reflect sunlight and thereby help reduce the Earth's albedo.
B. Volcanic eruptions increase the Earth's albedo.
1) only A
2) only B
3) both A and B
4) neither A nor B
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The table shows some characteristics for the planets of the solar system - Venus and Mars. It is known that the albedo of Venus A 1= 0.76, and the albedo of Mars A 2= 0.15. Which of the characteristics mainly influenced the difference in the albedo of the planets?
1) A 2) B 3) IN 4) G
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Does the Earth's albedo increase or decrease during volcanic eruptions? Explain your answer.
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Surface albedo refers to
1) total flux of solar rays incident on the Earth's surface
2) ratio of reflected radiation energy flux to absorbed radiation flux
3) ratio of reflected radiation energy flux to incident radiation flux
4) difference between incident and reflected radiation energy
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Study of spectra
All heated bodies emit electromagnetic waves. To experimentally study the dependence of radiation intensity on wavelength, it is necessary:
1) decompose the radiation into a spectrum;
2) measure the energy distribution in the spectrum.
Spectral devices - spectrographs - are used to obtain and study spectra. The diagram of the prism spectrograph is shown in the figure. The radiation under study first enters a tube, at one end of which there is a screen with a narrow slit, and at the other - a collecting lens L 1 . The slit is at the focal point of the lens. Therefore, a diverging light beam incident on the lens from the slit emerges from it as a parallel beam and falls on the prism R.
Since different frequencies correspond to different refractive indices, parallel beams of different colors come out of the prism, but do not coincide in direction. They fall on the lens L 2. At the focal length of this lens there is a screen, ground glass or photographic plate. Lens L 2 focuses parallel beams of rays on the screen, and instead of a single image of the slit, a whole series of images is obtained. Each frequency (more precisely, a narrow spectral interval) has its own image in the form of a colored stripe. All these images together
and form a spectrum.
Radiation energy causes the body to heat up, so it is enough to measure the body temperature and use it to judge the amount of energy absorbed per unit time. As a sensitive element, you can take a thin metal plate coated with a thin layer of soot, and by heating the plate, judge the radiation energy in a given part of the spectrum.
The decomposition of light into a spectrum in the apparatus shown in the figure is based on
1) phenomenon of light dispersion
2) phenomenon of light reflection
3) phenomenon of light absorption
4) properties of a thin lens
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In a prism spectrograph device, the lens L 2 (see figure) is used for
1) decomposition of light into spectrum
2) focusing rays of a certain frequency into a narrow strip on the screen
3) determination of radiation intensity in different parts of the spectrum
4) converting a diverging light beam into parallel rays
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Is it necessary to cover the metal plate of a thermometer used in a spectrograph with a layer of soot? Explain your answer.
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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.”
There are cold and hot air currents in the atmosphere. Where the warm layers are above the cold ones, air vortices are formed, under the influence of which the light rays are bent, and the position of the star changes.
The brightness of a star changes because rays that deviate incorrectly are concentrated unevenly over the surface of the planet. At the same time, the entire landscape is constantly shifting and changing due to atmospheric phenomena, for example, due to wind. The observer of the stars finds himself either in a more illuminated area, or, conversely, in a more shaded one.
If you want to watch the twinkling of stars, keep in mind that at the zenith, in a calm atmosphere, this phenomenon can only occasionally be detected. If you turn your gaze to celestial objects closer to the horizon, you will find that they twinkle much more. This is explained by the fact that you look at the stars through a denser layer of air, and, accordingly, penetrate a larger number of air currents with your gaze. You will not notice changes in the color of stars located at an altitude of more than 50°. But you will find frequent color changes in stars below 35°. Sirius flickers very beautifully, shimmering with all the colors of the spectrum, especially in the winter months, low above the horizon.
The strong twinkling of stars proves the heterogeneity of the atmosphere, which is associated with various meteorological phenomena. Therefore, many people think that flickering is related to the weather. It often gains strength at low atmospheric pressure, lower temperature, increased humidity, etc. But the state of the atmosphere depends on so many different factors that it is currently not possible to predict the weather from the twinkling stars.
This phenomenon keeps its mysteries and ambiguities. It is assumed that it intensifies at dusk. This could be an optical illusion or a consequence of unusual atmospheric changes that often occur at this time of day. It is believed that the twinkling of stars is caused by the northern lights. But this is very difficult to explain, given that the northern lights are located at an altitude of more than 100 km. In addition, it remains a mystery why white stars twinkle less than red ones.
Stars are suns. The first person to discover this truth was a scientist of Italian origin. Without any exaggeration, his name is known throughout the modern world. This is the legendary Giordano Bruno. He argued that among the stars there are similar to the Sun in size, temperature of their surface, and even color, which directly depends on temperature. In addition, there are stars that are significantly different from the Sun - giants and supergiants.
Table of ranks
The diversity of the countless stars in the sky forced astronomers to establish some order among them. To do this, scientists decided to divide the stars into appropriate classes of their luminosity. For example, stars that emit light several thousand times more than the Sun are called giants. In contrast, stars with minimal luminosity are dwarfs. Scientists have found that the Sun, according to this characteristic, is an average star.
do they light differently?
For a time, astronomers thought that stars shine differently because of their different locations from Earth. But it is not so. Astronomers have found that even those stars that are located at the same distance from the Earth can have completely different apparent brightness. This brightness depends not only on distance, but also on the temperature of the stars themselves. To compare stars by their apparent brightness, scientists use a specific unit of measurement - absolute magnitude. It allows us to calculate the real radiation of a star. Using this method, scientists have calculated that there are only 20 of the brightest stars in the sky.
Why are stars different colors?
It was written above that astronomers distinguish stars by their size and their luminosity. However, this is not their entire classification. Along with their size and apparent brightness, all stars are also classified according to their own color. The fact is that the light that defines this or that star has wave radiation. These are pretty short. Despite the minimum wavelength of light, even the smallest difference in the size of the light waves dramatically changes the color of the star, which directly depends on the temperature of its surface. For example, if you heat an iron frying pan, it will acquire the corresponding color.
The color spectrum of a star is a kind of passport that determines its most characteristic features. For example, the Sun and Capella (a star similar to the Sun) were identified by astronomers as one and the same. Both of them have a pale yellow color and a surface temperature of 6000°C. Moreover, their spectrum contains the same substances: lines, sodium and iron.
Stars such as Betelgeuse or Antares generally have a characteristic red color. Their surface temperature is 3000°C, and they contain titanium oxide. Stars such as Sirius and Vega are white. Their surface temperature is 10000°C. Their spectra have hydrogen lines. There is also a star with a surface temperature of 30,000°C - this is the bluish-white Orionis.
