1. Optical phenomena in the atmosphere were the first optical effects that have been observed by humans. With the understanding of the nature of these phenomena and the nature of human vision, the formation of the problem of light began.

Total number optical phenomena in the atmosphere is very large. Here only the most famous phenomena – mirages, rainbows, halos, crowns, twinkling stars, blue sky and scarlet color dawn. The formation of these effects is associated with such properties of light as refraction at the interfaces between media, interference and diffraction.

2. atmospheric refraction – is the curvature of light rays as they pass through the planet's atmosphere. Depending on the sources of rays, there are astronomical and terrestrial refraction. In the first case, the rays come from celestial bodies(stars, planets), in the second case - from terrestrial objects. As a result of atmospheric refraction, the observer sees an object not where it is, or not in the shape that it has.

3. Astronomical refraction was already known in the time of Ptolemy (2nd century AD). In 1604, I. Kepler suggested that earth atmosphere has a height-independent density and a certain thickness h(Fig. 199). Ray 1 coming from the star S straight to the observer A in a straight line, will not fall into his eye. Refracted at the boundary of vacuum and atmosphere, it will hit the point AT.

Ray 2 will hit the observer's eye, which, in the absence of refraction in the atmosphere, would have to pass by. As a result of refraction (refraction), the observer will see the star not in the direction S, but on the continuation of the beam refracted in the atmosphere, that is, in the direction S 1 .

Corner γ , which deviates to the zenith Z apparent position of the star S 1 vs. true position S, called refractive angle. At the time of Kepler, the angles of refraction were already known from results astronomical observations some stars. That's why this scheme Kepler used to estimate the thickness of the atmosphere h. According to his calculations, h» 4 km. If we count by the mass of the atmosphere, then this is about half the true value.

In fact, the density of the Earth's atmosphere decreases with altitude. Therefore, the lower layers of air are optically denser than the upper ones. Rays of light traveling obliquely to the Earth are not refracted at one point of the boundary between vacuum and atmosphere, as in Kepler's diagram, but are bent gradually along the entire path. This is similar to how a beam of light passes through a stack of transparent plates, the refractive index of which is the greater, the lower the plate is located. However, the total effect of refraction manifests itself in the same way as in the Kepler scheme. We note two phenomena due to astronomical refraction.

a. The apparent positions of celestial objects are shifting towards the zenith to the angle of refraction γ . The lower the star is to the horizon, the more noticeably its apparent position in the sky rises compared to the true one (Fig. 200). Therefore the picture starry sky, observed from the Earth, is somewhat deformed towards the center. Only the dot does not move S located at the zenith. Due to atmospheric refraction, stars that are slightly below the geometric horizon line can be observed.

Refraction angle values γ decrease rapidly as the angle increases. β the height of the luminary above the horizon. At β = 0 γ = 35" . This is the maximum angle of refraction. At β = 5º γ = 10" , at β = 15º γ = 3" , at β = 30º γ = 1" . For luminaries whose height β > 30º, refractive shift γ < 1" .

b. The sun illuminates more than half of the surface the globe . Rays 1 - 1, which in the absence of an atmosphere should touch the Earth at the points of the diametrical section DD, thanks to the atmosphere, they touch it a little earlier (Fig. 201).

The surface of the Earth is touched by rays 2 - 2, which would pass by without the atmosphere. As a result, the terminator line BB, separating light from shadow, shifts to the region of the night hemisphere. Therefore, the area of ​​the day surface on Earth is greater than the area of ​​the night.

4. Earth refraction. If phenomena astronomical refraction conditioned global refractive effect of the atmosphere, then the phenomena of terrestrial refraction are due local atmospheric changes usually associated with temperature anomalies. The most remarkable manifestations of terrestrial refraction are mirages.

a. superior mirage(from fr. mirage). It is usually observed in arctic regions with clear air and low surface temperatures. The strong cooling of the surface here is due not only to the low position of the sun above the horizon, but also to the fact that the surface covered with snow or ice reflects most radiation into space. As a result, in the surface layer, as it approaches the Earth's surface, the temperature decreases very quickly and increases optical density air.

The curvature of the rays towards the Earth is sometimes so significant that objects are observed that are far beyond the line of the geometric horizon. Beam 2 in Fig. 202, which in an ordinary atmosphere would have gone into its upper layers, into this case curves towards the Earth and enters the observer's eye.

Apparently, it is precisely such a mirage that represents the legendary “ Flying Dutchmen”- the ghosts of ships that are actually hundreds and even thousands of kilometers away. What is surprising in superior mirages is that there is no noticeable decrease in the apparent size of the bodies.

For example, in 1898 the crew of the Bremen ship "Matador" observed a ghost ship, the apparent dimensions of which corresponded to a distance of 3-5 miles. In fact, as it turned out later, this ship was at that time at a distance of about a thousand miles. (one nautical mile equal to 1852 m). Surface air not only bends light rays, but also focuses them as a complex optical system.

AT normal conditions air temperature decreases with increasing altitude. The reverse course of temperature, when the temperature rises with increasing altitude, is called temperature inversion. Temperature inversions can occur not only in arctic zones, but also in other, lower latitude places. Therefore, superior mirages can occur wherever the air is sufficiently clean and where temperature inversions occur. For example, distant vision mirages are sometimes observed on the coast mediterranean sea. Temperature inversion is created here by hot air from the Sahara.

b. inferior mirage occurs when reverse course temperatures and is usually observed in deserts during hot weather. By noon, when the sun is high, the sandy soil of the desert, consisting of particles of solid minerals, warms up to 50 degrees or more. At the same time, at a height of several tens of meters, the air remains relatively cold. Therefore, the refractive index of the above air layers is noticeably greater compared to the air near the ground. This also leads to beam bending, but in reverse side(fig.203).

Rays of light coming from the parts of the sky located low above the horizon, which are opposite the observer, are constantly bent upward and enter the eye of the observer in the direction from the bottom up. As a result, on their continuation on the surface of the earth, the observer sees a reflection of the sky, resembling a water surface. This is the so-called "lake" mirage.

The effect is even more enhanced when there are rocks, hills, trees, buildings in the direction of observation. In this case, they are visible as islands in the middle of a vast lake. Moreover, not only the object is visible, but also its reflection. By the nature of the curvature of the rays, the ground layer of air acts as a mirror of the water surface.

5. Rainbow. It's colorful an optical phenomenon observed during rain, illuminated by the sun and representing a system of concentric colored arcs.

The first theory of the rainbow was developed by Descartes in 1637. By this time, the following experimental facts related to the rainbow were known:

a. The center of the rainbow O is on the straight line connecting the Sun with the observer's eye.(fig.204).

b. Around the line of symmetry Eye - the Sun is a colored arc with an angular radius of about 42° . The colors are arranged, counting from the center, in the order: blue (d), green (h), red (k)(line group 1). it main rainbow. Inside the main rainbow there are faint multi-colored arcs of reddish and greenish hues.

in. The second system of arcs with an angular radius of about 51° called the secondary rainbow. Its colors are much paler and go into reverse order, counting from the center, red, green, blue (a group of lines 2) .

G. The main rainbow appears only when the sun is above the horizon at an angle of no more than 42 °.

As Descartes established, the main reason for the formation of the primary and secondary rainbows is the refraction and reflection of light rays in raindrops. Consider the main provisions of his theory.

6. Refraction and reflection of a monochromatic beam in a drop. Let a monochromatic beam with intensity I 0 falls on a spherical drop of radius R on distance y from the axis in the plane of the diametrical section (Fig. 205). At the point of fall A part of the beam is reflected, and the main part of the intensity I 1 passes inside the drop. At the point B most of the beam passes into the air (in Fig. 205 AT beam not shown), but minority reflected and falls to a point FROM. Stepped out at the point FROM beam intensity I 3 is involved in the formation of the main bow and weak secondary bands within the main bow.

Let's find the corner θ , under which the beam comes out I 3 with respect to the incident beam I 0 . Note that all the angles between the ray and the normal inside the drop are the same and equal to the angle of refraction β . (Triangles OAB and OVS isosceles). No matter how much the beam “circles” inside the drop, all angles of incidence and reflection are the same and equal to the angle of refraction β . For this reason, any ray emerging from the drop at the points AT, FROM etc., comes out at the same angle, equal to the angle fall α .

To find an angle θ beam deflection I 3 from the original, it is necessary to sum the deviation angles at points BUT, AT and FROM: q = (α – β) + (π – 2β) + (α - β) = π + 2α – 4β . (25.1)

More convenient to measure sharp corner φ \u003d π - q \u003d 4β – 2α . (25.2)

Having performed the calculation for several hundred rays, Descartes found that the angle φ with growth y, that is, as the beam moves away I 0 from the axis of the drop, first grows along absolute value, at y/R≈ 0.85 accepts maximum value and then starts decreasing.

Now this is the limit value of the angle φ can be found by examining the function φ to the extreme at. Since sin α = yçR, and sin β = yçR· n, then α = arcsin( yçR), β = arcsin( yçRn). Then

, . (25.3)

Expanding the terms into different parts of the equation and squaring, we get:

, Þ (25.4)

For yellow D-sodium lines λ = 589.3 nm refractive index of water n= 1.333. Point distance BUT occurrences of this ray from the axis y= 0,861R. The limiting angle for this ray is

Interesting that the point AT the first reflection of the beam in the drop is also the maximum distance from the drop axis. Exploring at an extreme angle d= p– α – ε = p– α – (p– 2β ) = 2β – α in size at, we get the same condition at= 0,861R and d= 42.08°/2 = 21.04°.

Figure 206 shows the dependence of the angle φ , under which the beam leaves the drop after the first reflection (formula 25.2), on the position of the point BUT beam entry into the drop. All rays are reflected inside a cone with an apex angle of ≈ 42º.

It is very important for the formation of a rainbow that the rays entering the drop in a cylindrical layer of thickness uçR from 0.81 to 0.90, come out after reflection in the thin wall of the cone in the angular range from 41.48º to 42.08º. Outside, the wall of the cone is smooth (there is an extremum of the angle φ ), from the inside - loose. The angular thickness of the wall is ≈ 20 arc minutes. For transmitted rays, the drop behaves like a lens with focal length f= 1,5R. Rays enter the drop over the entire surface of the first hemisphere, are reflected back by a diverging beam in the space of a cone with an axial angle of ≈ 42º, and pass through a window with an angular radius of ≈ 21º (Fig. 207).

7. The intensity of the rays emerging from the drop. Here we will speak only about the rays that emerged from the drop after the 1st reflection (Fig. 205). If a beam incident on a drop at an angle α , has intensity I 0 , then the beam that has passed into the droplet has an intensity I 1 = I 0 (1 – ρ ), where ρ is the intensity reflection coefficient.

For unpolarized light, the reflection coefficient ρ can be calculated using the Fresnel formula (17.20). Since the formula includes the squares of the functions of the difference and the sum of the angles α and β , then the reflection coefficient does not depend on whether the beam enters the droplet or from the droplet. Because the corners α and β at points BUT, AT, FROM are the same, then the coefficient ρ at all points BUT, AT, FROM the same. Hence, the intensity of the rays I 1 = I 0 (1 – ρ ), I 2 = I 1 ρ = I 0 ρ (1 – ρ ), I 3 = I 2 (1 – ρ ) = I 0 ρ (1 – ρ ) 2 .

Table 25.1 shows the values ​​of the angles φ , coefficient ρ and intensity ratios I 3 cI 0 calculated at different distances uçR beam entry for yellow sodium line λ = 589.3 nm. As can be seen from the table, when at≤ 0,8R into the beam I 3, less than 4% of the energy from the beam incident on the drop falls. And only starting from at= 0,8R and more up to at= R output beam intensity I 3 is multiplied.

Table 25.1
y/R	α ,º	β ,º	φ ,º	ρ	I 3 /I 0
0	0	0	0	0,020	0,019
0,30	17,38	12,94	16,99	0,020	0,019
0,50	29,87	21,89	27,82	0,021	0,020
0,60	36,65	26,62	33,17	0,023	0,022
0,65	40,36	29,01	35,34	0,025	0,024
0,70	44,17	31,52	37,73	0,027	0,025
0,75	48,34	34,09	39,67	0,031	0,029
0,80	52,84	36,71	41,15	0,039	0,036
0,85	57,91	39,39	42,08	0,052	0,046
0,90	63,84	42,24	41,27	0,074	0,063
0,95	71,42	45,20	37,96	0,125	0,095
1,00	89,49	48,34	18,00	0,50	0,125

So, the rays emerging from the drop at the limiting angle φ , have a much greater intensity compared to other beams for two reasons. Firstly, due to the strong angular compression of the beam of rays in the thin wall of the cone, and secondly, due to lower losses in the droplet. Only the intensity of these rays is sufficient to evoke in the eye a sensation of the brilliance of a drop.

8. Formation of the main rainbow. When light falls on a drop, the beam splits due to dispersion. As a result, the wall of the cone of bright reflection is stratified by colors (Fig. 208). purple rays ( l= 396.8 nm) exit at an angle j= 40°36", red ( l= 656.3 nm) - at an angle j= 42°22". In this angular interval D φ \u003d 1 ° 46 "encloses the entire spectrum of rays emerging from the drop. Violet rays form an inner cone, red ones form an outer cone. If the raindrops illuminated by the sun are seen by the observer, then those of them whose cone rays enter the eye are seen as the brightest. As a result, all drops that are in relation to the sun's ray passing through the observer's eye, at an angle of a red cone, are seen as red, at an angle of green - green (Fig. 209).

9. Secondary rainbow formation occurs due to the rays emerging from the drop after the second reflection (Fig. 210). The intensity of the rays after the second reflection is about an order of magnitude less than that of the rays after the first reflection and has approximately the same path with a change in uçR.

The rays emerging from the drop after the second reflection form a cone with an apex angle of ≈ 51º. If the primary cone has a smooth side on the outside, then the secondary cone has a smooth side on the inside. There are practically no rays between these cones. The larger the raindrops, the brighter the rainbow. With a decrease in the size of the droplets, the rainbow turns pale. When rain turns into drizzle R≈ 20 - 30 microns the rainbow degenerates into a whitish arc with almost indistinguishable colors.

10. Halo(from Greek. halōs- ring) - an optical phenomenon, which is usually iridescent circles around the disk of the sun or moon with an angular radius 22º and 46º. These circles are formed as a result of the refraction of light by ice crystals in cirrus clouds, which have the shape of hexagonal regular prisms.

Snowflakes falling to the ground are very diverse in shape. However, the crystals formed as a result of vapor condensation in the upper atmosphere are mainly in the form of hexagonal prisms. Of all options There are three most important passages of a beam through a hexagonal prism (Fig. 211).

In case (a), the beam passes through opposite parallel faces of the prism without splitting or deflecting.

In case (b), the beam passes through the faces of the prism, which form an angle of 60º between them, and is refracted as in a spectral prism. The intensity of the beam emerging at the angle of least deviation of 22º is maximum. In the third case (c), the beam passes through side face and the base of the prism. Refractive angle 90º, angle of least deviation 46º. In the latter two cases, the white rays are split, the blue rays deviate more, the red rays less. Cases (b) and (c) cause the appearance of rings observed in the transmitted rays and having angular dimensions of 22º and 46º (Fig. 212).

Usually the outer ring (46º) is brighter than the inner one and both of them have a reddish tint. This is explained not only by the intense scattering of blue rays in the cloud, but also by the fact that the dispersion of blue rays in the prism is greater than that of red ones. Therefore, blue rays leave the crystals in a strongly divergent beam, due to which their intensity decreases. And the red rays come out in a narrow beam, which has a much greater intensity. At favorable conditions when colors can be distinguished inner part rings red, outer - blue.

10. crowns- bright foggy rings around the disk of the star. Their corner radius is less than radius halo and does not exceed 5º. Crowns arise due to diffraction scattering of rays by water droplets forming a cloud or fog.

If the drop radius R, then the first diffraction minimum in parallel beams is observed at an angle j = 0,61∙lçR(see formula 15.3). Here l is the wavelength of the light. The diffraction patterns of individual drops in parallel beams coincide; as a result, the intensity of the light rings is enhanced.

The diameter of the crowns can be used to determine the size of the droplets in the cloud. The larger the drops (more R), the smaller the angular size of the ring. The largest rings are observed from the smallest droplets. At distances of several kilometers, diffraction rings are still visible when the droplet size is at least 5 µm. In this case j max = 0.61 lçR≈ 5 ¸ 6°.

The color of the light rings of the crowns is very weak. When it is noticeable, the outer edge of the rings has a reddish color. That is, the distribution of colors in the crowns is inverse to the distribution of colors in the halo rings. In addition to the angular dimensions, this also makes it possible to distinguish between the crowns and the halo. If there are droplets of a wide range of sizes in the atmosphere, then the rings of the crowns, superimposed on each other, form a general bright glow around the star's disk. This glow is called halo.

11. Blue sky and scarlet dawn. When the Sun is above the horizon, a cloudless sky appears blue. The fact is that from the rays of the solar spectrum, in accordance with the Rayleigh law I rass ~ 1 /l 4, short blue, cyan and violet rays are scattered most intensively.

If the Sun is low above the horizon, then its disk is perceived as crimson red for the same reason. Due to the intense scattering of short-wavelength light, mainly weakly scattered red rays reach the observer. Scattering of rays from the rising or setting Sun is especially great because the rays travel a long distance near the surface of the Earth, where the concentration of scattering particles is especially high.

Morning or evening dawn - coloring of the part of the sky close to the Sun in pink color- due to diffraction scattering of light by ice crystals in the upper atmosphere and geometric reflection crystal light.

12. twinkling stars- this is rapid change brilliance and color of stars, especially noticeable near the horizon. The twinkling of stars is due to the refraction of rays in rapidly running jets of air, which, due to different densities, have different indicator refraction. As a result, the layer of the atmosphere through which the beam passes behaves like a lens with a variable focal length. It can be both gathering and scattering. In the first case, the light is concentrated, the brilliance of the star is enhanced, in the second, the light is scattered. Such a sign change is recorded up to hundreds of times per second.

Due to dispersion, the beam is decomposed into rays of different colors, which follow different paths and can diverge the more, the lower the star is to the horizon. The distance between the violet and red rays from one star can reach 10 meters near the surface of the Earth. As a result, the observer sees a continuous change in the brightness and color of the star.

Phenomena due to refraction, reflection, scattering and diffraction of light in the atmosphere: from them one can conclude about the state of the corresponding layers of the atmosphere.

These include refraction, mirages, numerous halo phenomena, rainbows, crowns, dawn and dusk phenomena, blueness of the sky, etc.

Mirage(fr. mirage - lit. visibility) - an optical phenomenon in the atmosphere: the refraction of light streams at the boundary between layers of air that are sharply different in density and temperature. For an observer, such a phenomenon consists in the fact that, along with a really visible distant object (or a section of the sky), its reflection in the atmosphere is also visible.

Classification

Mirages are divided into lower, visible under the object, upper, visible above the object, and side.

inferior mirage

It is observed with a large vertical temperature gradient (its fall with height) over an overheated flat surface, often desert or paved road. The imaginary image of the sky creates the illusion of water on the surface. So, on a road that goes into the distance on a hot summer day, a puddle is seen.

superior mirage

It is observed above the cold earth's surface with an inverse temperature distribution (air temperature rises with altitude).

Superior mirages are generally less common than inferior mirages, but are often more stable because cold air does not tend to move up and warm tends to move down.

Superior mirages are most common in the polar regions, especially on large flat ice floes with stable low temperatures. Such conditions can occur over Greenland and around Iceland. Perhaps due to this effect, called hillingar(from Icelandic hillingar), the first settlers of Iceland became aware of the existence of Greenland.

Superior mirages are also observed at more moderate latitudes, although in these cases, they are fainter, less distinct and stable. An superior mirage can be upright or inverted, depending on the distance to the true object and the temperature gradient. Often the image appears as a fragmentary mosaic of upright and inverted parts.

A ship of normal size is moving beyond the horizon. In the specific state of the atmosphere, its reflection above the horizon seems gigantic.

Superior mirages may have striking effect due to the curvature of the earth. If the curvature of the rays is about the same as the curvature of the Earth, the light rays can travel long distances, causing the observer to see objects far beyond the horizon. This was observed and documented for the first time in 1596, when a ship under the command of Willem Barents, in search of the Northeast Passage, got stuck in the ice on Novaya Zemlya. The crew was forced to wait out the polar night. At the same time, the sunrise after polar night observed two weeks earlier than expected. In the 20th century, this phenomenon was explained and called the "New Earth Effect".

In the same way, ships that are actually so far away that they should not be visible above the horizon can appear on the horizon, and even above the horizon, as superior mirages. This may explain some of the stories about flights of ships or coastal cities in the sky, as described by some polar explorers.

side mirage

Lateral mirages can occur as a reflection from a heated sheer wall. A case is described when the smooth concrete wall of the fortress suddenly shone like a mirror, reflecting the surrounding objects. On a hot day, a mirage was observed whenever the wall was sufficiently heated by the sun's rays.

Fata Morgana

Complex phenomena of a mirage with a sharp distortion of the appearance of objects are called Fata Morgana. Fata Morgana(ital. fata morgana- fairy Morgana, according to legend, lives on seabed and deceiving travelers with ghostly visions) is a rare complex optical phenomenon in the atmosphere, consisting of several forms of mirages, in which distant objects are seen repeatedly and with various distortions.

Fata Morgana occurs when several alternating layers of air are formed in the lower layers of the atmosphere (usually due to temperature differences). different density able to give mirror reflections. As a result of reflection, as well as refraction of rays, in reality existing facilities they give several distorted images on the horizon or above it, partially overlapping each other and rapidly changing in time, which creates a bizarre picture of the Fata Morgana.

volumetric mirage

In the mountains it is very rare, under certain conditions, you can see a "distorted self" for quite close range. This phenomenon is explained by the presence of "stagnant" water vapor in the air.

Halo(from other Greek ἅλως - circle, disk; also aura, nimbus, halo) is an optical phenomenon, a luminous ring around a light source.

Physics of the phenomenon

The halo usually appears around the Sun or Moon, sometimes around other powerful light sources such as street lights. There are many types of halo and they are caused mainly by ice crystals in cirrus clouds at an altitude of 5-10 km in the upper troposphere. The appearance of the halo depends on the shape and location of the crystals. Light reflected and refracted by ice crystals is often decomposed into a spectrum, which makes the halo look like a rainbow. Parhelia and the zenith arc are the brightest and most full-color, while the tangents of the small and large halo are less bright. In a small 22-degree halo, only part of the colors of the spectrum (from red to yellow) is distinguishable, the rest looks white due to the repeated mixing of refracted rays. The parhelic circle and a number of other arcs of the halo are almost always white. An interesting feature of the large 46-degree halo is that it is dim and low-colored, while the upper tangent arc, which almost coincides with it at a low altitude of the Sun above the horizon, has pronounced iridescent colors.

In the dim lunar halo, colors are not visible to the eye, which is associated with the peculiarities of twilight vision.

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The atmosphere is a cloudy, optically inhomogeneous medium. Optical phenomena are the result of reflection, refraction and diffraction of light rays in the atmosphere.

Depending on the causes of occurrence, all optical phenomena are divided into four groups:

1) phenomena caused by the scattering of light in the atmosphere (twilight, dawn);

2) phenomena caused by the refraction of light rays in the atmosphere (refraction) - mirages, twinkling of stars, etc.;

3) phenomena caused by the refraction and reflection of light rays on drops and crystals of clouds (rainbow, halo);

4) phenomena caused by the diffraction of light in clouds and fog - crowns, gloria.

Twilight caused by the scattering of sunlight in the atmosphere. Twilight is the transitional period from day to night (evening twilight) and from night to day (morning twilight). Evening twilight begins from the moment the sun sets and until complete darkness sets in, morning twilight - vice versa.

The duration of twilight is determined by the angle between the direction of the apparent daily motion of the Sun and the horizon; thus, the duration of twilight depends on geographic latitude: the closer to the equator, the shorter the twilight.

There are three periods of twilight:

1) civil twilight (the immersion of the Sun under the horizon does not exceed 6 o) - light;

2) navigational (immersion of the Sun under the horizon up to 12 o) - visibility conditions are greatly deteriorated;

3) astronomical (immersion of the Sun under the horizon up to 18 o) - earth's surface it is already dark, but the dawn is still visible in the sky.

Dawn - a set of colorful light phenomena in the atmosphere, observed before sunrise or at sunset. The variety of colors of dawn depends on the position of the Sun relative to the horizon and on the state of the atmosphere.

The color of the firmament is determined by the scattered visible rays of the sun. In a clean and dry atmosphere, light scattering occurs according to Rayleigh's law. Blue rays scatter about 16 times more than red rays, so the color of the sky (scattered sunlight) is blue (blue), and the color of the Sun and its rays near the horizon is red, because. In this case, light travels a longer path in the atmosphere.

Large particles in the atmosphere (drops, dust particles, etc.) scatter light neutrally, so clouds and fog are white. With high humidity, dustiness, the entire sky becomes not blue, but whitish. Therefore, by the degree of blueness of the sky, one can judge the purity of the air and the nature of the air masses.

atmospheric refraction - atmospheric phenomena associated with the refraction of light rays. Refraction is due to: twinkling of stars, flattening of the visible disk of the Sun and Moon near the horizon, an increase in the length of the day by several minutes, as well as mirages. A mirage is a visible imaginary image on the horizon, above the horizon or below the horizon, due to a sharp violation of the density of air layers. There are inferior, superior, lateral mirages. Moving mirages - "Fata Morgana" are rarely observed.

Rainbow - this is a light arc, painted in all colors of the spectrum, against the background of a cloud illuminated by the Sun, from which raindrops fall. The outer edge of the arc is red, the inner edge is purple. If the Sun is low on the horizon, then we see only half of the circle. When the Sun is high, the arc becomes smaller, because. the center of the circle falls below the horizon. At a height of the Sun greater than 42 about the rainbow is not visible. From an airplane, you can observe a rainbow of almost a full circle.

A rainbow is formed by refraction and reflection sun rays in drops of water. The brightness and width of the rainbow depends on the size of the droplets. Large drops give a smaller but brighter rainbow. With small drops, it is almost white.

Halo - these are circles or arcs around the Sun and the Moon, arising in the ice clouds of the upper tier (most often in cirrostratus).

crowns - light, slightly colored rings around the Sun and Moon, arising in the water and ice clouds of the upper and middle tiers, due to the diffraction of light.

A person constantly encounters light phenomena. Everything that is connected with the appearance of light, its propagation and interaction with matter, is called light phenomena. Vivid examples of optical phenomena can be: a rainbow after rain, lightning during a thunderstorm, the twinkling of stars in the night sky, the play of light in a stream of water, the variability of the ocean and sky, and many others.

Students receive a scientific explanation of physical phenomena and optical examples in the 7th grade when they start studying physics. For many, optics will be the most fascinating and mysterious section in the school physics curriculum.

What does the person see?

Human eyes are designed in such a way that he can only perceive the colors of the rainbow. Today it is already known that the spectrum of the rainbow is not limited to red on one side and purple on the other. Per goes red infrared, behind violet is ultraviolet. Many animals and insects are able to see these colors, but unfortunately humans cannot. But on the other hand, a person can create devices that receive and emit light waves of the appropriate length.

refraction of rays

Visible light is a rainbow of colors, and light white color, for example, sunny, is a simple combination of these colors. If you place a prism in a beam of bright white light, it will break up into colors or waves. different lengths, of which it consists. First comes red with the longest wavelength, then orange, yellow, green, blue, and finally violet, which has the shortest wavelength in visible light.

If you take another prism to catch the light of the rainbow and turn it upside down, it will combine all the colors into white. There are many examples of optical phenomena in physics, let's consider some of them.

Why the sky is blue?

Young parents are often perplexed by the most simple, at first glance, questions of their little why. Sometimes they are the hardest to answer. Almost all examples of optical phenomena in nature can be explained by modern science.

The sunlight that illuminates the sky during the day is white, which means that, theoretically, the sky should also be bright white. In order for it to look blue, some processes with light are necessary at the time of its passage through the Earth's atmosphere. Here's what happens: some of the light passes through the free space between the gas molecules in the atmosphere, reaching the earth's surface and remaining the same white color as at the beginning of the journey. But sunlight encounters gas molecules, which, like oxygen, are absorbed and then dispersed in all directions.

The atoms in the gas molecules are activated by the absorbed light and again emit photons of light in waves various lengths- from red to purple. Thus, some of the light goes to the earth, the rest goes back to the sun. The brightness of the emitted light depends on the color. Eight photons of blue light are released for every photon of red. Therefore, blue light is eight times brighter than red. Intense blue light is emitted from all directions from billions of gas molecules and reaches our eyes.

colorful arch

Once upon a time, people thought that rainbows were signs sent to them by the gods. Indeed, beautiful multi-colored ribbons always appear in the sky from nowhere, and then just as mysteriously disappear. Today we know that the rainbow is one of the examples of optical phenomena in physics, but we do not cease to admire it every time we see it in the sky. The interesting thing is that each observer sees a different rainbow, created by rays of light coming from behind him and from raindrops in front of him.

What are rainbows made of?

The recipe for these optical phenomena in nature is simple: water droplets in the air, light and an observer. But it's not enough for the sun to come out when it rains. It should be low, and the observer should stand so that the sun is behind him, and look at the place where it is raining or just rained.

A sunbeam coming from distant space overtakes a raindrop. Acting like a prism, the raindrop refracts every color hidden in the white light. Thus, when white beam passes through a raindrop, it suddenly splits into beautiful multi-colored rays. Inside the drop, they hit the inner wall of the drop, which acts like a mirror, and the rays are reflected in the same direction from which they entered the drop.

The end result is a rainbow of colors arching across the sky - light bent and reflected by millions of tiny raindrops. They can act like small prisms, splitting white light into a spectrum of colors. But rain is not always necessary to see a rainbow. Light can also be refracted by fog or fumes from the sea.

What color is the water?

The answer is obvious - water has a blue color. If you pour pure water into a glass, everyone will see its transparency. This is because there is too little water in the glass and its color is too pale to see it.

When filling a large glass container, you can see the natural blue tint of the water. Its color depends on how water molecules absorb or reflect light. White light It is made up of a rainbow of colors, and water molecules absorb most of the red to green colors that pass through them. And the blue part is reflected back. So we see blue.

Sunrises and sunsets

These are also examples of optical phenomena that a person observes every day. When the sun rises and sets, it directs its rays at an angle to where the observer is. They have a longer path than when the sun is at its zenith.

The layers of air above the Earth's surface often contain a lot of dust or microscopic moisture particles. The sun's rays pass at an angle to the surface and are filtered. Red rays have the longest wavelength of radiation and therefore make their way to the ground more easily than blue rays, which have short waves that are beaten off by particles of dust and water. Therefore, during the morning and evening dawn, a person observes only a part of the sun's rays that reach the earth, namely red ones.

planet light show

A typical aurora is a multi-colored aurora in the night sky that can be observed every night at the North Pole. Shifting in bizarre shapes, huge streaks of blue-green light flecked with orange and red sometimes reach over 160 km in width and can stretch for 1,600 km in length.

How to explain this optical phenomenon, which is such a breathtaking sight? Auroras appear on Earth, but they are caused by processes occurring on the distant Sun.

How is everything going?

The sun is a huge ball of gas, consisting mainly of hydrogen and helium atoms. All of them have protons with a positive charge and electrons revolving around them with negative charge. A halo of hot gas constantly spreads into space in the form solar wind. This countless number of protons and electrons are rushing at a speed of 1000 km per second.

When solar wind particles reach the Earth, they are attracted by a strong magnetic field planets. Earth is a giant magnet with magnetic lines that converge at the North and south poles. The attracted particles flow along these invisible lines near the poles and collide with the nitrogen and oxygen atoms that make up the Earth's atmosphere.

Some of the earth's atoms are losing their electrons, others are being charged new energy. After colliding with the protons and electrons of the Sun, they give off photons of light. For example, nitrogen that has lost electrons attracts violet and blue light, while charged nitrogen shines dark red. Charged oxygen gives off green and red light. Thus, the charged particles cause the air to shimmer with many colors. This is the aurora borealis.

Mirages

It should immediately be determined that mirages are not a figment of human imagination, they can even be photographed, they are almost mystical examples of optical physical phenomena.

There is a lot of evidence of the observation of mirages, but science can give a scientific explanation for this miracle. They can be as simple as a patch of water amid hot sands, or they can be stunningly complex, constructing visions of pillared castles or frigates. All these examples of optical phenomena are created by the play of light and air.

Light waves bend as they pass first through warm, then cold air. Hot air is more rarefied than cold air, so its molecules are more active and diverge over greater distances. As the temperature decreases, the movement of molecules also decreases.

Visions seen through the lenses of the earth's atmosphere can be highly altered, compressed, expanded, or inverted. This is because light rays bend as they pass through warm and then cold air, and vice versa. And those images that a light stream carries with it, for example, the sky, can be reflected on hot sand and seem like a piece of water, which always moves away when approached.

Most often, mirages can be observed at great distances: in deserts, seas and oceans, where hot and cold layers of air with different density. It is the passage through different temperature layers that can twist light wave and end up with a vision that is a reflection of something and presented by fantasy as a real phenomenon.

Halo

For most optical illusions that can be seen with the naked eye, the explanation is the refraction of the sun's rays in the atmosphere. One of the most unusual examples of optical phenomena is solar halo. Basically, a halo is a rainbow around the sun. However, it differs from the usual rainbow both in appearance and in its properties.

This phenomenon has many varieties, each of which is beautiful in its own way. But for the occurrence of any kind of this optical illusion certain conditions are required.

A halo occurs in the sky when several factors coincide. Most often it can be seen in frosty weather with high humidity. In the air, there is a large number of ice crystals. Breaking through them, the sunlight is refracted in such a way that it forms an arc around the Sun.

And although the last 3 examples of optical phenomena are easily explained by modern science, for an ordinary observer they often remain mystic and a mystery.

Having considered the main examples of optical phenomena, it can be safely assumed that many of them are explained by modern science, despite their mysticism and mystery. But scientists still have a lot of discoveries, clues ahead. mysterious phenomena that take place on planet Earth and beyond.

Lyceum Petru Movila

Course work in physics on the topic:

Optical atmospheric phenomena

The work of a student of grade 11A

Bolyubash Irina

Chisinau 2006 -

Plan:

1. Introduction

a) What is optics?

b) Types of optics

2. Earth's atmosphere as an optical system

3. sunny sunset

a) sky color change

b) Sun rays

in) The uniqueness of sunsets

4. Rainbow

a) rainbow formation

b) Variety of rainbows

5. auroras

a) Types of auroras

b) Solar wind as the cause of auroras

6. Halo

a) light and ice

b) Prism crystals

7. Mirage

a) Explanation of the lower ("lake") mirage

b) superior mirages

in) Double and triple mirages

G) Mirage of ultra-long vision

e) Legend of the Alps

e) Parade of superstitions

8. Some mysteries of optical phenomena

Introduction

What is optics?

The first ideas of ancient scientists about light were very naive. It was believed that special thin tentacles come out of the eyes and visual impressions arise when they feel objects. At that time, optics was understood as the science of vision. This is the exact meaning of the word "optics". In the Middle Ages, optics gradually turned from the science of vision into the science of light. This was facilitated by the invention of lenses and the camera obscura. In modern times, optics is a branch of physics that studies the emission of light, its propagation in various media and interaction with matter. As for issues related to vision, the structure and functioning of the eye, they stood out in a special scientific direction called physiological optics.

The term "optics" modern science, is multifaceted. These are atmospheric optics, and molecular optics, and electron optics, and neutron optics, and nonlinear optics, and holography, and radio optics, and picosecond optics, and adaptive optics, and many other phenomena and methods scientific research closely related to optical phenomena.

Most of the listed types of optics, as a physical phenomenon, are available to our observation only when using special technical devices. It can be laser systems, X-ray emitters, radio telescopes, plasma generators and many more. But the most accessible and, at the same time, the most colorful optical phenomena are atmospheric. Huge in scale, they are the product of the interaction of light and the atmosphere of the earth.

Earth's atmosphere as an optical system

Our planet is surrounded gas envelope which we call the atmosphere. Possessing the greatest density at the earth's surface and gradually rarefied as it rises, it reaches a thickness of more than a hundred kilometers. And it's not frozen gaseous environment with identical physical data. Conversely, the earth's atmosphere is in constant motion. Under influence various factors, its layers mix, change density, temperature, transparency, move long distances at different speeds.

For rays of light coming from the sun or other celestial bodies, the earth's atmosphere is a kind of optical system with constantly changing parameters. Being in their way, it reflects part of the light, scatters it, passes it through the entire thickness of the atmosphere, providing illumination of the earth's surface, under certain conditions, decomposes it into components and bends the path of the rays, thereby causing various atmospheric phenomena. The most unusual colorful ones are sunset, rainbow, Northern Lights, mirage, solar and lunar halo.

sunny sunset

The simplest and most accessible atmospheric phenomenon to observe is the sunset of our heavenly body- Sun. Extraordinarily colorful, it never repeats itself. And the picture of the sky and its change in the process of sunset is so bright that it arouses admiration in every person.

Approaching the horizon, the Sun not only loses its brightness, but also begins to gradually change its color - the short-wavelength part (red colors) is increasingly suppressed in its spectrum. At the same time, the sky begins to color. In the vicinity of the Sun, it acquires yellowish and orange tones, and a pale stripe with a weakly expressed gamut of colors appears above the antisolar part of the horizon.

By the time of sunset, which has already taken on a dark red color, a bright band of dawn stretches along the solar horizon, the color of which changes from bottom to top from orange-yellow to greenish-blue. A round, bright, almost uncolored radiance spreads over it. At the same time, at the opposite horizon, a bluish-gray dim segment of the Earth's shadow begins to slowly rise, bordered by a pink belt. ("Girdle of Venus").

As the Sun sinks deeper below the horizon, a rapidly spreading pink spot appears - the so-called "purple light" reaching greatest development at a depth of the Sun under the horizon of about 4-5 o . Clouds and mountain tops are flooded with scarlet and purple tones, and if clouds or high mountains are beyond the horizon, their shadows stretch about sunny side the sky and become more saturated. Near the horizon, the sky turns red, and across the brightly colored sky, light rays stretch from horizon to horizon in the form of distinct radial stripes. ("Rays of the Buddha"). Meanwhile, the shadow of the Earth is rapidly moving into the sky, its outlines becoming blurry, and the pink border is barely noticeable. Gradually, the purple light fades, the clouds darken, their silhouettes stand out distinctly against the background of the fading sky, and only at the horizon, where the Sun has hidden, is a bright multi-colored segment of dawn preserved. But it also gradually shrinks and turns pale, and by the beginning of astronomical twilight turns into a greenish-whitish narrow strip. Finally, she disappears - the night comes.

The described picture should be considered only as typical for clear weather. In fact, the nature of the sunset flow is subject to wide variations. With increased air turbidity, the colors of dawn are usually faded, especially near the horizon, where instead of red and orange tones, sometimes only a faint brown color appears. Quite often, simultaneous glow phenomena develop differently in different parts of the sky. Each sunset has a unique personality and this should be considered one of their most characteristic features.

The extreme individuality of the sunset flow and the variety of optical phenomena accompanying it depend on various optical characteristics of the atmosphere - primarily its attenuation and scattering coefficients, which manifest themselves differently depending on the zenith distance of the Sun, the direction of observation and the height of the observer.

Rainbow

Rainbow is beautiful celestial phenomenon has always attracted people's attention. AT old times, when people still knew little about the world around them, the rainbow was considered a "heavenly sign." So, the ancient Greeks thought that the rainbow is the smile of the goddess Irida.

The rainbow is observed in the direction opposite to the Sun, against the background of rain clouds or rain. A multi-colored arc is usually located at a distance of 1-2 km from the observer, and sometimes it can be observed at a distance of 2-3 m against the background of water drops formed by fountains or water sprays.

The center of the rainbow is on the continuation of the straight line connecting the Sun and the eye of the observer - on the anti-solar line. The angle between the direction to the main rainbow and the anti-solar line is 41º - 42º

At the time of sunrise, the antisolar point is on the horizon line, and the rainbow looks like a semicircle. As the sun rises, the antisolar point falls below the horizon and the size of the rainbow decreases. It is only part of a circle.

Often there is a secondary rainbow, concentric with the first, with an angular radius of about 52º and an inverse arrangement of colors.

The main rainbow is formed by the reflection of light in water droplets. A secondary rainbow is formed as a result of a double reflection of light inside each drop. In this case, the rays of light exit the drop at different angles than those that produce the main rainbow, and the colors in the secondary rainbow are in reverse order.

The path of rays in a drop of water: a - with one reflection, b - with two reflections

At a Sun height of 41º, the main rainbow ceases to be visible and only a part of the secondary rainbow appears above the horizon, and at a Sun height of more than 52º, the secondary rainbow is not visible either. Therefore, in the middle equatorial latitudes, this natural phenomenon is never observed during the near noon hours.

The rainbow has seven primary colors that smoothly transition from one to another. The shape of the arc, the brightness of the colors, the width of the stripes depend on the size of the water droplets and their number. Large drops create a narrower rainbow, with sharply prominent colors, small drops create an arc that is blurry, faded and even white. That's why bright narrow rainbow visible in the summer after a thunderstorm, during which large drops fall.