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Light phenomena physics 7. Abstract: Light phenomena

Allows you to determine the location and movement of the planets, the Sun, the Moon and other luminaries. We observe light phenomena in nature everywhere. Our eyes help us in this, as well as special devices that make it possible to learn about the structure of celestial bodies, even those that are at a distance of billions of kilometers from the Earth. Observations through a telescope and photography of the planets made it possible to study the cloud cover, rotation speed, surface features.

The nature of planet Earth gives us unique, rare, beautiful and incredible natural phenomena.

Varieties of lighting effects

Here are just a few of them:

circumhorizontal arc. It is also called "fiery rainbow". When light passes through the ice crystals of cirrus clouds, the sky is covered with colored stripes, and the sky seems to be covered with a "rainbow film". Such light phenomena are very rare, since a natural phenomenon occurs only when ice crystals and the sun's rays in relation to each other are at a certain angle.

Rainbow clouds. This effect also depends on how the Sun is located to the water droplets from the clouds. Colors are determined by different wavelengths of light.

"Ghost of the Brocken". Amazing light phenomena are observed in some areas of our planet: if the sun sets or rises behind a person standing on a hill or mountain, he may find that his shadow, which falls on the clouds, increases to improbable sizes. This is due to the refraction of the sun's rays by the smallest drops of fog. Such an effect is regularly observed at the top of the Brocken in Germany.

Halo. Sometimes white circles appear around the Moon and Sun. This occurs as a result of the reflection or refraction of light by snow or ice crystals. In frosty weather, halos, which are formed by snow and ice crystals on the ground, reflect light and scatter it in different directions, resulting in an effect called "diamond dust".

Parhelion. The word "parhelion" means "false sun". It is a kind of halo: there are several additional Suns in the sky, located at a level with the present one.

Everyone knows such an atmospheric phenomenon as a rainbow, which occurs after rain - the most beautiful atmospheric phenomenon.

Northern Lights. Similar light phenomena are observed in the polar regions. It is assumed that the same phenomenon exists in the atmosphere of other planets, Venus, for example. Scientists believe that the auroras result from the bombardment of the upper atmospheric layer by charged particles that move towards the Earth parallel to the geomagnetic field lines from outer space, called the plasma layer.

Polarization is the orientation in space of electromagnetic oscillations of light waves. This phenomenon occurs when light strikes a surface at a certain angle and becomes polarized upon reflection. Such a sky can be seen using a camera filter.

Star track. The phenomenon can be captured by a camera, but it is impossible to do it with the naked eye.

The corona around the Sun is the small colored crowns around a given planet or bright objects. They are occasionally observed in cases where light sources are hidden behind translucent clouds, and occurs when light rays are scattered by small water droplets that form a cloud.

Mirage - this optical effect, which is due to the refraction of light rays when passing through layers of air with different densities. It is expressed by the appearance of a deceptive image. Mirages are most often observed in hot climates, mainly in deserts. Sometimes they display entire objects that are at a great distance from the observer.

Pillars of light. These are such light phenomena when light is reflected from ice crystals, and vertical luminous pillars are formed, as if emerging from the surface of the earth. The source in this case is the Moon, the Sun or artificial lights.

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  • Participant: Maksimova Anna Alekseevna
  • Head: Gusarova Irina Viktorovna

Objective - to study light phenomena and the properties of light in experiments, to consider the three main properties of light: straightness of propagation, reflection and refraction of light in media of different density.

Tasks:

  1. Prepare equipment.
  2. Carry out the necessary experiments.
  3. Analyze and present the results.
  4. Make a conclusion.

Relevance

In everyday life, we are constantly confronted with light phenomena and their various properties; the work of many modern mechanisms and devices is also associated with the properties of light. Light phenomena have become an integral part of people's lives, so their study is relevant.

The experiments below explain such properties of light as straightness of propagation, reflection and refraction of light.

For providence and description of experiments, the 13th stereotyped edition of A. V. Peryshkin's textbook “Physics. 8th grade." (Drofa, 2010)

Safety

The electrical devices involved in the experiment are fully operational, the voltage on them does not exceed 1.5 V.

The equipment is stably placed on the table, the working order is observed.

At the end of the experiments, electrical appliances are turned off, the equipment is removed.

Experience 1. Rectilinear propagation of light. (p. 149, fig. 120), (p. 149, fig. 121)

Purpose of experience- to prove the rectilinearity of the propagation of light rays in space using a good example.

The rectilinear propagation of light is its property, which we encounter most often. With rectilinear propagation, energy from a light source is directed to any object along straight lines (light rays), without bending around it. This phenomenon can explain the existence of shadows. But in addition to shadows, there are also penumbra, partially illuminated areas. To see under what conditions shadows and penumbras are formed and how light propagates in this case, we will conduct an experiment.

Equipment: an opaque sphere (on a thread), a sheet of paper, a point light source (a flashlight), an opaque sphere (on a thread) smaller in size, for which the light source will not be a point, a sheet of paper, a tripod for fixing the spheres.

Experience progress

Shadow formation
  1. Let's arrange the objects in the order pocket flashlight-first sphere (fixed on a tripod)-sheet.
  2. Let's get the shadow displayed on the sheet.

We see that the result of the experiment was a uniform shadow. Suppose that the light propagated in a straight line, then the formation of a shadow can be easily explained: the light coming from a point source along the light beam, touching the extreme points of the sphere, continued to go in a straight line and behind the sphere, which is why the space behind the sphere is not illuminated on the sheet.

Let's assume that the light propagated along curved lines. In this case, the rays of light, bending, would also fall outside the sphere. We would not have seen the shadow, but as a result of the experiment, the shadow appeared.

Now consider the case in which penumbra is formed.

Formation of shade and penumbra
  1. Let's arrange the objects in the order pocket flashlight-second sphere (fixed on a tripod)-leaf.
  2. Illuminate the sphere with a flashlight.
  3. Let's get a shadow, as well as a penumbra, displayed on the sheet.

This time the results of the experiment are shadow and penumbra. How the shadow was formed is already known from the example above. Now, in order to show that the formation of penumbra does not contradict the hypothesis of rectilinear propagation of light, it is necessary to explain this phenomenon.
In this experiment, we took a light source that is not a point, that is, consisting of many points, in relation to a sphere, each of which emits light in all directions. Consider the highest point of the light source and the light beam emanating from it to the lowest point of the sphere. If we observe the movement of the beam behind the sphere to the sheet, then we will notice that it falls on the border of light and penumbra. Rays from similar points going in this direction (from the point of the light source to the opposite point of the illuminated object) create penumbra. But if we consider the direction of the light beam from the above indicated point to the upper point of the sphere, then it will be perfectly visible how the beam enters the penumbra region.

From this experience we see that the formation of penumbra does not contradict the rectilinear propagation of light.

Conclusion

With the help of this experiment, I proved that light propagates in a straight line, the formation of a shadow and penumbra proves the rectilinearity of its propagation.

Phenomenon in life

The straightness of light propagation is widely used in practice. The simplest example is an ordinary lantern. Also, this property of light is used in all devices that include lasers: laser rangefinders, metal cutting devices, laser pointers.

In nature, the property is found everywhere. For example, light penetrating through gaps in the crown of a tree forms a well-defined straight line passing through the shadow. Of course, if we talk about large scales, it is worth mentioning a solar eclipse, when the moon casts a shadow on the earth, due to which the sun from the earth (of course, we are talking about its shaded area) is not visible. If the light did not propagate in a straight line, this unusual phenomenon would not exist.

Experience 2. Law of reflection of light. (p.154, fig. 129)

Purpose of experience- prove that the angle of incidence of the beam is equal to the angle of its reflection.

Reflection of light is also its most important property. It is thanks to the reflected light, which is captured by the human eye, that we can see any objects.

According to the law of light reflection, the rays, incident and reflected, lie in the same plane with a perpendicular drawn to the interface between two media at the point of incidence of the beam; the angle of incidence is equal to the angle of reflection. Let's check whether these angles are equal, in an experiment, where we take a flat mirror as a reflecting surface.

Equipment: a special device, which is a disk with a printed circular scale, mounted on a stand, in the center of the disk there is a small flat mirror located horizontally (such a device can be made at home using a protractor instead of a disk with a circular scale), the light source is an illuminator attached to the edge of the disc or laser pointer, measurement sheet.

Experience progress

  1. Let's place the sheet behind the device.
  2. Turn on the illuminator, directing it to the center of the mirror.
  3. Let's draw a perpendicular to the mirror to the point of incidence of the beam on the sheet.
  4. Let us measure the angle of incidence (ﮮα).
  5. Let us measure the resulting reflection angle (ﮮβ).
  6. Let's write down the results.
  7. Let's change the angle of incidence by moving the illuminator, repeat steps 4, 5 and 6.
  8. Let's compare the results (the value of the angle of incidence with the value of the angle of reflection in each case).

The results of the experiment in the first case:

∠α = 50°

∠β = 50°

∠α = ∠β

In the second case:

∠α = 25°

∠β = 25°

∠α = ∠β

It can be seen from experience that the angle of incidence of a light beam is equal to the angle of its reflection. Light hitting a mirror surface is reflected from it at the same angle.

Conclusion

With the help of experience and measurements, I proved that when light is reflected, the angle of its incidence is equal to the angle of reflection.

Phenomenon in life

We encounter this phenomenon everywhere, as we perceive the light reflected from objects with the eye. A striking visible example in nature is the glare of bright reflected light on water and other surfaces with good reflectivity (the surface absorbs less light than it reflects). Also, one should remember the sunbeams that every child can let out with the help of a mirror. They are nothing more than a ray of light reflected from a mirror.

A person uses the law of reflection of light in such devices as a periscope, a mirror reflector of light (for example, a reflector on bicycles).

By the way, by reflecting light from a mirror, magicians created many illusions, for example, the “Flying Head” illusion. The man was placed in a box among the scenery so that only his head was visible from the box. The walls of the box were covered with mirrors inclined towards the scenery, the reflection from which did not allow the box to be seen and it seemed that there was nothing under the head and it was hanging in the air. The sight is unusual and frightening. Reflection tricks also took place in theaters when a ghost had to be shown on the stage. The mirrors were "fogged" and tilted so that the reflected light from the niche behind the stage was visible in the auditorium. An actor playing a ghost has already appeared in the niche.

Experience 3. Refraction of light.(p. 159, fig. 139)

Purpose of experience- prove that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant value for two media; prove that the angle of incidence of a light beam (≠ 0°) coming from a less dense medium to a denser one is greater than its angle of refraction.

In life, we often meet with the refraction of light. For example, putting a perfectly straight spoon into a transparent glass of water, we see that its image is bent at the border of two media (air and water), although in fact the spoon remains straight.

To better consider this phenomenon, to understand why it occurs and to prove the law of refraction of light (rays, incident and refracted, lie in the same plane with a perpendicular drawn to the interface between two media at the point of incidence of the beam; the ratio of the sine of the angle of incidence to the sine of the angle of refraction is the value is constant for two media) using an example, we will conduct an experiment.

Equipment: two media of different density (air, water), a transparent container for water, a light source (laser pointer), a sheet of paper.

Experience progress

  1. Pour water into a container, place a sheet behind it at some distance.
  2. Let us direct a beam of light into water at an angle, ≠ 0°, since at 0° there is no refraction, and the beam passes into another medium unchanged.
  3. Let us draw a perpendicular to the interface between two media at the point of incidence of the beam.
  4. Let us measure the angle of incidence of the light beam (∠α).
  5. Let us measure the angle of refraction of the light beam (∠β).
  6. Let's compare the angles, make up the ratio of their sines (to find the sines, you can use the Bradis table).
  7. Let's write down the results.
  8. Let's change the angle of incidence by moving the light source, repeat steps 4-7.
  9. Let's compare the values ​​of the sine ratios in both cases.

Let us assume that light rays, passing through media of different densities, experienced refraction. In this case, the angles of incidence and refraction cannot be equal, and the ratios of the sines of these angles are not equal to one. If no refraction has occurred, that is, the light has passed from one medium to another without changing its direction, then these angles will be equal (the ratio of the sines of equal angles is equal to one). To confirm or refute the assumption, consider the results of the experiment.

The results of the experiment in the first case:

∠α = 20

∠β = 15

∠α >∠β

sin∠α = 0.34 = 1.30

sin∠β 0.26

The results of the experiment in the second case:

∠α ˈ= 50

∠β ˈ= 35

∠α ˈ > ∠β ˈ

sin∠α ˈ= 0.77 = 1.35

sin∠β ˈ 0.57

Comparison of sine ratios:

1.30 ~1.35 (due to measurement errors)

sin∠α = sin∠α ˈ = 1.3

sin∠β sin∠β ˈ

According to the results of the experiment, when light is refracted from a less dense medium to a denser one, the angle of incidence is greater than the angle of refraction. the ratios of the sines of the incident and refracted angles are equal (but not equal to one), that is, they are a constant value for the two given media. The direction of the beam when it enters a medium of a different density changes due to a change in the speed of light in the medium. In a denser medium (here, in water), light propagates more slowly, and therefore the angle of passage of light through space changes.

Conclusion

With the help of experience and measurements, I proved that when light is refracted, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant value for both media, when light rays pass from a less dense medium to a denser one, the angle of incidence is less than the angle of refraction.

Phenomenon in life

With the refraction of light, we also meet quite often, we can give many examples of the distortion of the visible image when passing through water and other media. The most interesting example is the occurrence of a mirage in the desert. A mirage occurs when light rays passing from warm layers of air (less dense) to cold layers are refracted, which can often be observed in deserts.

Human refraction of light is used in various devices containing lenses (light is refracted when passing through a lens). For example, in optical instruments such as binoculars, a microscope, a telescope, in cameras. Also, a person changes the direction of light by passing it through a prism, where the light is refracted several times, entering and leaving it.

The objectives of the work have been achieved.

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Physics abstract

on the topic: "Light phenomena in nature"

pupils of 8 "L 1" class

Introduction

Why is the sky blue and the sunset is red

Tyndall experience

light scattering

Air fluctuation

green beam

"Blind" lane

Refraction

Ice crystals in the clouds

Halo in Antarctica

superior mirages

"Ghost" lands

"Flying Dutchman"

inferior mirages

Side mirages

Fata Morgana

misty rainbow

moon rainbow

auroras

Types of auroras

Influence of the aurora

Conclusion

Bibliography

Introduction

How amazing is nature! Light phenomena look especially unusual and bewitching. Since ancient times, people have perceived this as a miracle, linking the inexplicable with mystical forces or with the gods.

It became interesting to me: after all, there is an explanation for all these unusual phenomena. And I decided, in a new way, from a physical point of view, to look at some light phenomena and find answers to many interesting questions.

The sun is the source of energy for plant and animal life. It creates winds by heating vast land masses and air masses above them, and serves as the driving force behind the water cycle in nature, raising water vapor into the atmosphere. The sun is a vital component of the environment, without which life on Earth would not be possible.

The sun's rays illuminate the entire globe. Beautiful world of sunlight. He brings joy to all living on Earth. The world of sunlight is vast, diverse, inexhaustible.

The sky is infinitely beautiful, and light phenomena also look beautiful and amazing: sunset, “blind streak”, green beam, rainbow, northern lights, halo, mirages. In this essay, phenomena inextricably linked with Sunlight will be considered, many wonders of nature will be explained.

Whythe sky is blue and the sunset is red

The sun... Already in ancient times, people understood that without the sun's rays, life on Earth would be impossible. They called the sun "the beginning of life", deified it, worshiped it. At all times, the sunset caused sadness, fear, anxiety in people, but more often the sunset inspires a slight sadness, bordering on peace. The observed picture of the sunset depends each time on the state of the atmosphere and is largely determined by the type and shape of the clouds illuminated by the rays of the setting sun. Therefore, one sunset is so different from the other. And the sunsets are always extraordinarily beautiful.

First of all, the reddish color of the setting Sun and the same color of the sky near it are striking. It is redder near the horizon line, and in the upper part of the disk it turns into a color of lighter tones.

Tyndall experience

The sky is blue and the color of the setting sun turns red. In both cases, the reason is the same - the scattering of sunlight in the earth's atmosphere. This can be explained by assuming that blue light is scattered more than red light. This was proved in 1869 when J. Tyndall carried out his famous experiment. This experience is not at all difficult to replicate. Let's take a rectangular aquarium, fill it with water and direct a weakly diverging narrow beam of light from a slide projector onto the aquarium wall. The experiment should be carried out in a darkened room. To enhance the scattering of the light beam as it passes through the aquarium, add some milk to the water. The fat particles contained in milk do not dissolve in water; they are in suspension, and contribute to the scattering of light. A bluish tint can be observed in scattered light. The light that has passed through the aquarium acquires a reddish tint. So, if you look at the light beam in the aquarium from the side, it appears bluish, and reddish from the exit end. This can be explained by the fact that when a white light beam passes through a scattering medium, it is mainly the "blue component" that scatters from it; therefore, the "red component" begins to predominate in the beam leaving the medium.

light scattering

In 1871, J. Strett explained the results of Tyndall's experiments in exactly this way. He developed a theory of the scattering of light waves by particles whose dimensions are much smaller than the wavelength of light. Rayleigh's law states: The scattered light intensity is proportional to the fourth power frequency of light, or, in other words, is inversely proportional to the fourth power of the wavelength of light.

If Rayleigh's law is applied to the scattering of sunlight in the Earth's atmosphere, then it is not difficult to explain both the blue color of the daytime sky and the red color of the sun at sunrise and sunset. Since light waves with higher frequencies are scattered more intensively, then, consequently, the spectrum of the scattered light will be shifted towards higher frequencies, and the spectrum of the light remaining in the beam, after the scattered light has left the beam, will be shifted in the opposite direction - to more low frequencies. In the first case, the white color becomes blue, and in the second, it becomes reddish. Looking at the daytime sky, the observer perceives the light scattered in the atmosphere; according to Rayleigh's law, the spectrum of this light is shifted towards higher frequencies, hence the blue color of the sky. Looking at the sun, the observer perceives light that has passed through the atmosphere without scattering; the spectrum of this color will be shifted to low frequencies. The closer the sun is to the horizon line, the longer the light rays travel in the atmosphere before reaching the observer, the more their spectrum shifts. As a result, we see the setting and rising sun in red tones. It is also quite understandable why the lower part of the setting solar disk looks more red than its upper part.

The main role is played by the dependence of the light scattering intensity on its frequency. But what is the nature of those centers on which light waves scatter? Initially, it was thought that the role of such centers is played by the smallest dust particles and droplets of water, but this does not explain the wonderful blue color of the sky in high mountain regions, where the air is very clean and dry.

Air fluctuation

In 1899, Rayleigh put forward a hypothesis according to which the centers that scatter light are the air molecules themselves. In the first half of the 20th century, thanks to the works of M. Smoluchovsky, A. Einstein and L. I. Mandelstam, it was found that, in reality, light scattering occurs not on the air molecules themselves, but on somewhat unusual objects arising from the chaotic movement of the thermal motion of molecules , - on fluctuations in air density, i.e., randomly occurring microscopic concentrations and rarefaction of air. We see that some cells are almost empty, and some are relatively densely populated with molecules. This is a consequence of the chaotic thermal motion of air molecules. As a result, the density of atmospheric air will randomly change (fluctuate) from one cell to another. It is clear that at a different moment in time, already different cells will be more or less populated, but the air density will still change randomly. It is possible to explain the concept of air density fluctuations in another way. Let's focus not on any particular moment in time, but on some arbitrarily chosen cell of space. Over time, the number of molecules in a cell will fluctuate where several different time points are considered. Simply put, the density of the air at a given point will randomly change over time. These local air density inhomogeneities are the scattering centers that determine the blue color of the daytime sky and the red color of the setting sun. The presence of fine dust and water droplets in the air leads to additional scattering and to some extent affects the color of the sky and sunset. However, the root cause is the scattering of light by fluctuations in air density. The nature of these fluctuations largely depends on the state of the atmosphere: the temperature of the various layers of air, the nature and strength of the wind. That is why the sunset is golden in calm clear weather, and crimson in windy weather.

green beam

An amazing sight - a green beam. A bright green light flashes for a few seconds when almost the entire solar disk has disappeared below the horizon. This can be seen on such evenings when the Sun shines brightly until sunset and almost does not change its color. It is important that the horizon has a distinct line without any irregularities: forests, buildings, etc. These conditions are easiest to achieve at sea.

The appearance of the green beam can be explained if we take into account the change in the refractive index with the frequency of light. Typically, the refractive index increases with increasing frequency. Rays with a higher frequency are refracted more strongly. This means that blue-green rays undergo a stronger refraction compared to red rays.

Let us assume that there is refraction of light in the atmosphere, but there is no scattering of light. In this case, the upper and lower edges of the solar disk near the horizon would have to be colored in the colors of the rainbow. Let for simplicity in the spectrum of sunlight there are only two colors - green and red; The “white” solar disk can be considered in this case as green and red disks superimposed on each other. The refraction of light in the atmosphere raises the green disk above the horizon to a greater extent than the red one. The top edge of the solar disk would be green and the bottom red; in the central part of the disk, a mixture of colors would be observed, i.e., a white color would be observed.

In reality, the scattering of light in the atmosphere cannot be ignored. As we already know, it leads to the fact that rays with a higher frequency drop out of the light beam coming from the sun more efficiently. So we will not see a green border on top of the disk, and the entire disk will look not white, but reddish. If, however, almost the entire solar disk has gone below the horizon, only its uppermost edge remains, and at the same time the weather is clear and calm, the air is clean (so light scattering is minimal), then in this case we can see the bright green edge of the sun along with a scattering of bright, green rays. And yet we will see green, because blue has dissipated in the atmosphere.

"Blind» strip

Another amazing phenomenon: sometimes the Sun seems to set not behind a clearly visible horizon line, but behind some invisible line above the horizon. Interestingly, this phenomenon is observed in the absence of any cloud cover. If you quickly climb to the top of the hill, you can observe an even stranger picture: now the sun is setting behind the horizon line, but at the same time the solar disk appears to be cut as if by a horizontal blind stripe. The sun gradually sinks lower and lower, and the position of the blind strip in relation to the horizon remains unchanged.

A sunset picture is observed if the air near the earth's surface turns out to be cold enough, and there is a layer of relatively warm air above. In this case, the refractive index of air changes with height a) The transition from the lower cold layer of air to the warm layer above it can lead to a rather sharp drop in the refractive index. For simplicity, we will assume that this decline occurs abruptly, so there is a clearly defined interface between the cold and warm layers, located at a certain height h0 above the earth's surface. The aforementioned jump b) where nx denotes the refractive index of air in the cold layer, and nt denotes the refractive index of air in the warm layer near the boundary with the cold one.

Refraction

The time of sunrise and sunset in any place of the globe on any day of the year is calculated according to astronomical formulas quite accurately. But in fact, the calculated time of sunrise and sunset and the actual time do not always coincide. The fact is that the atmosphere surrounding the Earth makes its own “corrections”.

Air density decreases rapidly with altitude. Along with the density, the refractive index and the propagation velocity of electromagnetic waves in the atmosphere change.

refraction called the refraction of electromagnetic waves in the atmosphere due to the inhomogeneity of the air density, both in the horizontal and, especially strongly, in the vertical directions. The trajectories of electromagnetic waves in the atmosphere are complex curves.

A direct consequence of the refraction of the sun's rays is an increase in the length of the day. At sunset, when its disk has already sunk below the horizon, its refraction raises it, and the day continues. Similarly, at sunrise: the Sun is still under the horizon, and due to refraction, we already see it, that is, the day begins before the actual sunrise.

The increase in the length of the day depends on the latitude of the place and the declination of the Sun on a given day. In middle latitudes, due to refraction, the day usually increases by no more than 8 - 12 minutes. If we move along the earth's surface towards the poles, then the lengthening of the day becomes more and more significant. At the poles of the globe, where the polar day and polar night should last exactly half a year, it turns out that the polar day is 14 days longer than the polar night.

Halo

If the Sun or Moon shines through thin cirrostratus clouds consisting of ice crystals, light phenomena called halo often appear in the sky. Halo phenomena are very diverse.

At moments close to sunset or sunrise, columns of light appear above the Sun, and sometimes below it.

The repeatability of the halo is determined by the frequency of occurrence of cirrostratus clouds. Often several forms of halo are observed simultaneously in the sky. A complex complex of various halos was observed in St. Petersburg on July 18, 1794. 12 different circles and arcs were simultaneously observed in the sky, 9 of them were colored. Other complex halos that have been observed in different parts of the globe are also described.

The appearance in the sky at the same time of several suns, light crosses, oblique arcs, which, especially during dawn, seemed to be “bloody swords”, in former times caused fear in people, gave rise to superstition, was perceived as a harbinger of great misfortune - war, famine.

Ice crystals in the clouds

How do halos appear? All forms of halo are the result of the refraction of solar or lunar rays in the ice crystals of the cloud, or their reflection from the side faces or bases of the crystals, which are in the form of hexagonal columns or plates. Strictly speaking, diffraction of solar or lunar rays occurs on crystals.

Halo in Antarctica

Most often, various halos occur at inland stations located on the ice dome of Antarctica and on its slope at altitudes of 2700 - 3500 m above sea level.

In the absence of dense snow clouds, when the Sun shines, unusually bright colored and white halos appear. Often only the lower halves of halo circles are visible.

Halos in Antarctica are often observed throughout the day, only their shape and brightness of colors change.

Another interesting light phenomenon, which was seen only in the depths of the Antarctic continent, is the rainbow, or colored, drifting snow. It is observed only at a low position of the Sun, and in order to better see it, one must lie on the snow and look towards the Sun. Wind-blown trickles of drifting snow, meeting snowfalls on their way, fly up, forming small and large multi-colored fountains flashing with all the colors of the rainbow.

Colored drifting snow occurs as a result of the refraction of sunlight in the hollow ice crystals that make up the drifting snow, and in the crystals settling from the clouds. The origin of the colored drifting snow is similar to the “play” of light in crystal chandeliers, pendants, and diamond jewelry.

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 really exists on Earth, often enlarged and greatly distorted. A mirage can be sketched, photographed, filmed, which has been done many times. There are several types of mirages depending on where the image is located in relation to the subject. Mirages are: upper, lower, side and complex. refraction solar drift fluctuation

The most frequently observed superior and inferior mirages occur with an unusual distribution of density along the height, when at a certain height or near the very surface of the Earth there is a relatively thin layer of very warm air, in which the rays coming from ground objects experience total internal reflection.

superior mirages

In superior mirages, the image is located above the subject. Such mirages occur when air density and refractive index decrease rapidly with altitude.

Horizon expansion is often observed over cold seas or over chilled land surfaces. The earth, as it were, straightens a little, and very distant objects rise from behind the horizon and become visible.

"Ghostly"hearth

The number of upper mirages, apparently, should include at least part of the so-called ghost lands, which have been searched for decades in the Arctic and have not been found. These are the Lands of Andreev, Gilles, Oscar, Sannikov and others. The search for Sannikov Land was especially long.

In 1811, Sannikov set out 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 could not reach it - huge polynyas interfered. Sannikov announced the discovery of a new island to the tsarist government. In August 1886 E.V. Tol during his expedition to the New Siberian Islands also saw Sannikov Island.

Tol spent 16 years of his life searching for Sannikov Land. He organized and conducted three expeditions to the area of ​​the New Siberian Islands. During the last expedition on the schooner Zarya, Tolya's expedition perished without finding Sannikov Land. No one else has seen Sannikov Land. Perhaps it was a mirage that appears in the same place at certain times of the year. Both Sannikov and Tol saw the mirage of the same island located in this direction, only much further in the ocean.

The English polar explorer Robert Scott suggested in 1902 that there was a mountain range further beyond the horizon. Indeed, the mountain range was later discovered by the Norwegian polar explorer Roald Amundsen and exactly where Scott had supposed it to be.

"Flying Dutchman"

The Flying Dutchman is a ghostly sailing ship of unusually large size with no visible crew on board. It suddenly appeared, silently walked, not responding to signals, and just as suddenly disappeared. The meeting with the "Flying Dutchman" was considered fatal, it was necessary to wait for a storm or other trouble.

It was, without a doubt, an upper mirage, that is, an image of an ordinary sailing ship that calmly sailed somewhere far beyond the horizon, and its enlarged and distorted image, in the form of an upper mirage, rose into the air, and it was taken for " Flying Dutchman. Mirage, of course, did not respond to any signals from other ships. Now the Flying Dutchman, in the form of a sailing ship, has disappeared from the seas and oceans, as sailing ships have become rare. You can see the mirages of ships sailing beyond the visible horizon quite often.

inferior mirages

Inferior mirages occur when temperature decreases very rapidly with height. The mirage is called the lower one, since the image of the object is placed under the object. In lower mirages, it seems as if there is a water surface under the object, and all objects are reflected in it.

Reflection in a thin layer of air heated from the earth's surface is completely analogous to reflection in water. Only the air itself plays the role of a mirror. The state of air in which inferior mirages occur is extremely unstable. After all, below, near the ground, lies strongly heated, which means lighter air, and above it - colder and heavier. 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 waving. A small gust of wind or a push is enough and collapse will occur, that is, the air layers will turn over. 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, even underlying surface of the Earth, which takes place in the steppes and deserts, and sunny calm weather.

The apparent water surface or lake seen in a mirage is in reality a reflection of the sky. Parts of the sky are reflected in the air mirror and create a complete illusion of a brilliant water surface. Such mirages are visible in summer, on sunny days over asphalt roads or a flat sandy beach.

Side mirages

Lateral mirages can occur when layers of air of the same density are located obliquely or even vertically in the atmosphere. Such conditions are created in the summer, in the morning shortly after sunrise near 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. A side mirage can appear at the stone wall of a house heated by the Sun, and even to the side of a heated stove.

Fata Morgana

Mirages of a complex type, or veil - Morgana, arise when there are conditions for the appearance of both the upper and lower mirages at the same time. Air density first increases with height and then also rapidly decreases. 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 is changing before our eyes. The most ordinary rocks and houses, due to repeated distortions and magnification, turn into the wonderful castles of Fairy Morgana before our eyes.

Rainbow

A commonly observed rainbow is a colored arc seen against the backdrop of a shower curtain or rainfall streaks, often not reaching the ground. The rainbow is visible in the side of the sky opposite the Sun, and always with the Sun not covered by clouds. Such conditions are most often created during summer heavy rains.

Most people who have observed a rainbow many times do not see, or rather do not notice additional arcs in the form of the most delicate colored arches inside the first and outside the second rainbows. These colored arcs are incorrectly called complementary - in fact, they are as basic as the first and second rainbows. These arcs do not form a whole semi-circle or great arc, and are only visible in the uppermost parts of the rainbow. It is in these arcs, and not in the main ones, that the greatest wealth of pure color tones is concentrated.

All rainbows are sunlight broken down into its components and moved across the firmament in such a way that it appears to come from the part of the firmament opposite that of the Sun.

The whole view of the rainbow - the width of the arcs, the presence, location and brightness of individual color tones, the position of additional arcs very much depend on the size of the raindrops.

By the appearance of a rainbow, one can approximately estimate the size of the raindrops that formed this rainbow. In general, the larger the raindrops, the narrower and brighter the rainbow, especially for large drops is the presence of saturated red in the main rainbow. Numerous additional arcs also have bright colors and directly, without gaps, adjoin the main rainbows. The smaller the droplets, the wider and faded the rainbow with an orange or yellow edge. From the surface of the Earth, we can observe a rainbow at best in the form of a half circle when the Sun is on the horizon. From the plane you can observe the rainbow in the form of a whole circle.

misty rainbow

White rainbows are found in nature. They appear when sunlight illuminates a faint fog, consisting of droplets with a radius of 0.025 mm or less. They are called foggy rainbows. In addition to the main rainbow in the form of a brilliant white arc with a barely noticeable yellowish edge, sometimes colored additional arcs are observed: a very faint blue or green arc, and then a whitish red. A similar kind of white rainbow can be seen when a spotlight behind you illuminates an intense haze or faint fog in front of you. Even a street lamp can create, albeit a very faint, white rainbow visible against the dark background of the night sky.

moon rainbow

Like solar rainbows, lunar rainbows can also occur. They are weaker and appear during the full moon. Lunar rainbows are rarer than solar rainbows. For their occurrence, a combination of two conditions is necessary: ​​a full moon, not covered by clouds, and the fall of heavy rain. Lunar rainbows can be observed anywhere in the world where the above two conditions are met.

Daytime, solar rainbows, even formed by the smallest drops of rain or fog, are rather whitish, light, and yet their outer edge, at least slightly, is colored orange or yellow. Rainbows formed by moon rays do not live up to their name at all, since they are not iridescent and look like light, completely white arcs.

The lack of red color in the lunar arcs, even with large drops of heavy rain, is explained by the low level of illumination at night, at which the sensitivity of the eye to red rays is completely lost. The remaining color rays of the rainbow also lose their color tone to a large extent due to the lack of color in human night vision.

auroras

Auroras are flashes of light in the form of bright colored bands. Auroras occur when electrons and protons flying from space collide with atoms and molecules in the upper atmosphere. The collision results in the emission of light - sometimes white, but more often green and red. After a solar flare, auroras are always brighter and can be observed at latitudes closer to the equator.

The ancient Romans called the goddess of the morning dawn Aurora. With her name, they also associated the auroras, occasionally observed in the middle latitudes. After all, like the morning dawn, these radiances were painted in pink and red colors. With the light hand of the Romans, the term "aurora" was later applied to the auroras. At present, this term is also fixed in the scientific literature; all phenomena associated with auroras are now called auroral events.

Types of auroras

The aurora borealis is always an unusually majestic spectacle. Polar lights are very diverse. But with all the diversity, several specific forms can be distinguished. Usually there are four main forms.

The simplest form is homogeneous arc (uniform band). It has a fairly even glow, brighter at the bottom of the arc and gradually disappearing at the top. The arc usually extends across the entire sky in the direction east - west; its length reaches thousands of kilometers, while its thickness is only a few kilometers. The length of the luminous band in the vertical direction is measured in hundreds of kilometers; the lower edge of the strip is, as a rule, at altitudes of 100-150 km. Homogeneous arcs (stripes) are whitish-green, as well as reddish or purple.

The next form of aurora -- rays . Narrow vertical luminous lines closely lined up one after another are visible in the sky, as if many powerful searchlights placed in a row shine upwards. For an observer who looks at the radiance not from the side, but directly from below, the rays appear to converge at the top (perspective effect). Starting from a height of about 100 km, the rays go up hundreds and even thousands of kilometers. Together they form a radiant band. It is usually greenish in color; at the bottom, the band often has a pinkish-orange border.

A particularly strong impression is made by glows having the form tapes , which can form folds or twist into peculiar spirals. Giant curtains hang high in the sky, they sway, wave, change shape and brightness. The thickness of these curtains is about a kilometer; in height they are located approximately from 100 to 400 km. The color of the ribbons is mainly greenish-blue, with a transition to pinkish and red tones in the lower part.

Finally, it should be noted aurora, having the form of blurry spots , similar to giant luminous clouds; they are called diffuse spots. A separate such spot has an area of ​​about 100 km². As a rule, the spots are painted in whitish or reddish tones. They are formed at altitudes of about 100 km, as well as at altitudes of 400...500 km. Various forms of auroras can occur simultaneously, overlapping one another.

Rays, ribbons, spots are not at all motionless: they move and, at the same time, the intensity of their glow changes over time. The speed of the beams and ribbons can reach tens of kilometers per second. During the night, one can observe the gradual transformation of some forms of aurora into others. For example, a homogeneous arc can suddenly break into rays or turn into folds of a ribbon, and the latter can then break up into cloud-like spots.

Influencepolarradiance

At one time, the appearance of auroras was associated with tragic phenomena in nature and society. Is it only the fear of incomprehensible impressive natural phenomena that underlies these superstitions? It is now well known that solar rhythms with different periods (27 days, 11 years, etc.) affect the most diverse aspects of life on Earth. Solar and magnetic storms (and associated auroras) can cause an increase in various diseases, including diseases of the human cardiovascular system. Changes in the climate on Earth, the occurrence of droughts and floods, earthquakes, etc. are associated with solar cycles. All this makes us once again seriously think about the connection between the auroras and earthly cataclysms and troubles. Maybe the old ideas about such a connection are not so stupid?

Aurora borealis signal the place and time of the influence of the Cosmos on earth processes. The invasion of charged particles that causes them affects many aspects of our lives. The ozone content and the electric potential of the ionosphere change, the heating of the ionospheric plasma excites waves in the atmosphere. All this affects the weather. Due to additional ionization, significant electric currents begin to flow in the ionosphere, the magnetic fields of which distort the Earth's magnetic field, which directly affects the health of many people. Thus, through the auroras and the processes associated with them, the Cosmos affects the nature around us and its inhabitants.

Conclusion

Writing an essay was entertaining and interesting: I did not just present information, but also learned interesting things with interest.

After writing the essay, I learned about some phenomena that I had never seen. Now I will watch the sky more often: I really want to see some phenomena, the explanation of which I already know. Of particular interest were such things as the green beam, the "blind" streak and mirages. And some phenomena have ceased to be incomprehensible to me: after all, there is an explanation for everything from a physical point of view, it’s just that not everything has been studied yet.

I learned why the sky is blue, how and on what light is scattered in the atmosphere, what fluctuation is, how a rainbow is formed, and much more. But there are still many mysteries in nature, no less interesting.

Bibliography

1. Tarasov "Physics in nature"

2. Ian Nicholson translated by V. N. Mikhailov Encyclopedia "Universe"

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Literature: ; ;

Optics (from the Greek word opticos - visual) is a branch of physics that studies the nature of light, the laws of light phenomena and the processes of interaction of light with matter.

During the last three centuries, the idea of ​​the nature of light has undergone a very significant change. At the end of the XVII century. two fundamentally different theories about the nature of light were formed: corpuscular theory, developed by Newton, and wave theory, developed by Huygens. According to the corpuscular theory, light is a stream of material particles (corpuscle), flying at high speed from a light source.

According to the wave theory, light is a wave emanating from a light source and propagating at high speed in the "world ether" - a motionless elastic medium that continuously fills the entire Universe. Thus, the wave theory considered light as mechanical waves propagating in a special medium (like sound waves in air).

Until the end of the XVIII century. the vast majority of physicists preferred Newton's corpuscular theory. At the beginning of the 19th century, thanks to the research cabin boy and Fresnel wave theory has been largely developed and improved. The wave theory successfully explained almost all light phenomena known at that time, including interference, diffraction and polarization of light, in connection with which this theory received universal recognition, and Newton's corpuscular theory was rejected.

The weak point of the wave theory was the hypothetical "world ether", the reality of whose existence remained very doubtful (in 1881, the American physicist Michelson experimentally proved that the world ether does not exist). In the 60s of the XIX century, when Maxwell developed the theory of a unified electromagnetic field, the need for the "world ether" as a special carrier of light waves disappeared: it turned out that light is electromagnetic waves and, therefore, their carrier is an electromagnetic field.

visible light corresponds to electromagnetic waves with a length of 0.77 to 0.38 microns, created by vibrations of charges that make up atoms and molecules. Thus, the wave theory about the nature of light evolved into electromagnetnew theory of light.

The idea of ​​the wave (electromagnetic) nature of light remained unshakable until the end of the 19th century. By that time, however, quite extensive material had accumulated that did not agree with this idea and even contradicted it.

The study of data on the emission spectra of chemical elements, on the distribution of energy in the spectrum of thermal radiation of a black body, on the photoelectric effect and some other phenomena led to the need to assume that the emission and absorption of electromagnetic energy is discrete(discontinuous) character, i.e., light is not emitted and absorbed continuously (as followed from the wave theory), but portions (quanta).

Based on this assumption, the German physicist Planck in 1900 he created the quantum theory of electromagnetic processes, and Einstein in 1905 developed quantum theory of light according to which light is a stream light particles- photons. However, photons differ significantly (qualitatively) from ordinary material particles: all photons move at a speed equal to the speed of light, while possessing ultimate mass (“rest mass” of a photon is equal to zero).

An important role in the further development of the quantum theory of light was played by the theoretical studies carried out by Borom, Schrödinger, Dirac, Feynman, fokom and others. According to modern views, light- a complex electromagnetic process that has both wave and corpuscular propertiesyou.

In some phenomena (interference, diffraction, polarization of light) the wave properties of light are revealed; these phenomena are described by the wave theory. In other phenomena (the photoelectric effect, luminescence, atomic and molecular spectra), the corpuscular properties of light are revealed; such phenomena are described by quantum theory.

Thus, wave (electromagnetic) and corpuscular (quantum) theory do not reject, but complement each other, thereby reflecting dual nature of the properties of light. Here we meet with a clear example of the dialectical unity of opposites: light is both a wave and a particle. It is appropriate to emphasize that such dualism is inherent not only in light, but also in microparticles of substances.

Modern physics seeks to create single the theory of the nature of light, reflecting the dual corpuscular-wave nature of light; the development of such a unified theory has not yet been completed.

Light interference is a phenomenon of amplification or weakening of oscillations, which occurs as a result of the addition of two or more waves converging at a certain point in space. A necessary condition for the interference of waves is their coherence: the equality of their frequencies and a time-constant phase difference. This condition satisfy monochromatic light waves (from the Greek (monos) - one, (chroma) - color, i.e. any one wavelength corresponds to monochromatic light). If this condition is met, the interference of other waves (for example, sound) can also be observed.

For light waves, as well as for any other, the principle of superposition is valid. Since light is electromagnetic in nature, the application of this principle means that the resulting strength of the electric (magnetic) field of two light waves passing through one point is equal to the vector sum of the strengths of the electric (magnetic) fields of each of the waves separately.

In a particular case, when the strengths of the constituent fields are equal but oppositely directed, the strength of the resulting field will be equal to zero (light is extinguished by light). If they are directed in one direction, there is a maximum amplification of light.

The result of interference is an interference pattern - a time-stable distribution of interference maxima and minima in space (for example, the alternation of dark and light stripes on the screen; in nature, the iridescent color of the wings of insects and birds, soap bubbles, oil film on water, etc.).

A special case of the interference pattern are the so-called Newton's rings (Figure 4.1)

Figure 4.1

They are observed in a system formed by a plane-parallel plate and a plano-convex lens with a large radius of curvature in contact with it.

The result of the interference of two light waves (in the same medium) depends on the path difference Δl=l 1 -l 2 (Figure 4.2).

Figure 4.2

If an even number of half-waves fits into the difference in the path of the rays, i.e. if

(4.1.1)

then at point A on the screen there will be a maximum of light (λ is the wavelength, S 1 and S 2 are monochromatic light sources, n = 0,1,2,3, ...). If an odd number of half-waves fit into the difference in the path of the rays, i.e. if

(4.1.2)

then at point A there will be a minimum of light. The interference pattern created by two coherent light sources on the screen is an alternation of dark and light stripes.

The interference pattern is very sensitive to the difference between the paths of the interfering waves. Based on this interferometer device an instrument used to determine small lengths, angles, the refractive index of the medium, and the lengths of light waves.

Diffraction is the deviation of light from a rectilinear propagation near an obstacle (light bending around an obstacle). So, for example, if another screen B with a hole is placed between the light source S and screen A, on screen A one can observe a diffraction pattern consisting of alternating light and dark rings and capturing the geometric shadow area (especially noticeable when the size of the hole is much smaller than the distance between the screens).

Figure 4.3

When white (non-monochromatic light) is used, the diffraction pattern becomes iridescent.

The phenomenon of diffraction is explained using the Huygens-Fresnel principle. According to this principle, each point of the wave surface reaching the hole becomes a secondary light source. These sources are coherent, so the light rays coming from them will interfere with each other. Depending on the magnitude of the path difference, maxima and minima of illumination will appear on screen A. In laboratory practice, the diffraction pattern is usually obtained from narrow luminous slits. The set of a large number of parallel narrow slits transparent to light, separated by opaque gaps, is called grating. Diffraction gratings are made by drawing thin strokes on the surface of a glass plate (transparent grating) or a metal mirror (reflective). The sum of the slot width a and the gap b between the slots is called the period or lattice constant: d = a + b. Diffraction gratings give a clear diffraction pattern and are used to determine the wavelength, as well as in spectral analysis to decompose light into a spectrum and infer the chemical composition of a substance. Diffraction patterns often occur in nature. For example, colored rings surrounding a light source when the air is saturated with water droplets (fog) or dust are the result of light diffraction on these particles. Diffraction explains the color of mother-of-pearl and the iridescent color of the eyes of many insects, whose eyes are a kind of diffraction gratings.

In chemistry, X-ray diffraction analysis, a method for studying the structure of a substance by distribution in space and the intensities of X-ray radiation scattered on the analyzed object, has received wide application. It is based on the interaction of X-ray radiation with the electrons of matter, which results in the diffraction of X-rays. The diffraction pattern depends on the wavelength of the X-rays used and the structure of the object. To study the atomic structure, radiation with a wavelength of the order of the size of atoms is used. Metals, alloys, minerals, inorganic and organic compounds, polymers, amorphous materials, liquids and gases, molecules of proteins, nucleic acids, etc. are studied by the methods of X-ray diffraction analysis. It is most successfully used to establish the atomic structure of crystalline bodies. This is due to the fact that crystals have a strict periodicity of the structure and represent a diffraction grating for X-rays created by nature itself.

Light represents the total electromagnetic radiation of many atoms. As you know, an electromagnetic wave can be represented as oscillations of two mutually perpendicular lecturers of electric E and magnetic H strengths. Since the electromagnetic wave is transverse, both vectors oscillate in planes perpendicular to the velocity vector - the direction of propagation of the beam. An electromagnetic wave in which only one of these vectors oscillates is impossible. An electric field in which E changes inevitably generates a magnetic field in which H changes according to the same law, and vice versa. The polarization phenomena are considered relative to the intensity vector E, but one should remember about the obligatory existence of the intensity vector H perpendicular to it. The plane in which the electric field intensity vector oscillates is called the oscillation plane. The plane in which the magnetic field strength vector oscillates is called the plane of polarization.

Natural light from this point of view can be schematically represented as follows (Figure 4.4):

Figure 4.4

The uniform arrangement of the vectors E is due to the large number of atomic emitters. This light is called unpolarized. In such light waves, the vectors have different orientations of vibrations, and all orientations are equally probable. If the influence of external influences on the light or the internal features of the light source appears the preferred, most probable direction of oscillation, then such light is called partially polarized(Figure 4.5) .

Figure 4.5

With the help of special devices, a beam can be selected from a beam of natural light, in which the oscillations of the vector E will occur in one specific plane (Figure 4.6)

Figure 4.6

Such light will be completely polarized. Unlike natural light, polarized light is characterized, in addition to intensity and wavelength, by the position of the plane of polarization. The human eye does not distinguish between natural and polarized light. In practice, polarized light is usually obtained by passing natural light through crystals, which are known to be anisotropic (the physical properties depend on the direction in the crystal). Polarized light is widely used in chemical and biological research. For example, some substances, called optically active, rotate the plane of polarization of polarized light passing through them. Moreover, the angle of rotation depends on the thickness of the substance layer. Thus, it is possible to determine the concentration of substances in a solution, which underlies the method of studying substances - polarimetry. With the help of optical polarimeters, the magnitude of the rotation of the plane of polarization of light is determined when it passes through optically active media (solids or solutions). Polarimetry is widely used in analytical chemistry to quickly measure the concentration of optically active substances to identify essential oils and in other studies. Almost all biologically functional molecules are optically active.

An important optical characteristic of the medium is absolute refractive index n (or just the refractive index). It shows how many times the speed of light in a given medium is less than the speed of light in a vacuum

(4.1.3)

The value of the refractive index of a medium is mainly determined by the properties of this medium. However, to some extent it also depends on the wavelength (frequency) of light. Therefore, the same medium refracts light rays of different wavelengths in different ways. The dependence of the refractive index of a medium on the wavelength of light is called the dispersion of light (from the Latin dispersio - scattering).

The dispersion is called normal if the refractive index increases with decreasing light wave, otherwise it is anomalous. Due to dispersion, a beam of white light passing through a refractive medium is decomposed into various monochromatic beams (red, orange, yellow, green, cyan, indigo, violet). Getting on the screen, these rays form a dispersion spectrum - a set of multi-colored bands. The dispersion spectrum is most clearly detected when light is refracted in a prism (Figure 4.7).

Figure 4.7

The angle D between the rays corresponding to the extreme colors of the dispersion spectrum is called the dispersion angle. It depends on the width of the spectrum. By the form of the spectrum, one can judge the chemical composition of the refractive medium. The so-called spectral analysis is based on this.

When light passes through a substance, it is partially absorbed due to the conversion of the electromagnetic energy of a light wave into other types of energy (for example, thermal energy). Substances that absorb light weakly are called transparent. Strongly absorbing light - opaque. Such a division is relative, since transparency depends not only on the type of substance, but also on the thickness of its layer. In addition, the absorption of light by a substance is selective. Different substances absorb light of different wavelengths differently. This is what determines the color of the body. From a stream of white color, this body absorbs only rays of a certain wavelength, the rest are transmitted, reflected or scattered and perceived by the human eye. So, for example, the leaves of living plants have significant absorption in the entire visible spectrum, except for the green and dark red parts of it.

When light propagates in a homogeneous medium, as studies by Bouguer and Lambert have shown, the light intensity changes according to the following law:

(4.1.4)

where I 0 is the light intensity at the entrance to the substance layer, I is the light intensity at the exit from it, x is the thickness of the substance layer, k is the absorption coefficient, depending on the type of substance and wavelength. The absorption of light is ultimately responsible for all types of light effects on matter. It is as a result of the action of light that photosynthesis occurs (the transformation of inorganic substances into organic ones, accompanied by the release of oxygen).

Passing through a turbid medium (a medium in which many particles of any foreign substance are suspended), light diffracts from its randomly located microhomogeneities and spreads in all directions (scatters). In this case, the medium acquires a blue tint. This phenomenon is explained by the Rayleigh law:

I~1/λ 4 (4.1.5)

those. the scattered light intensity is inversely proportional to the fourth power of the wavelength. From formula (4.1.4) it can be seen that rays with a shorter wavelength are scattered more strongly (blue light has the smallest wavelength). Scattering of light also occurs in media purified from foreign particles (the so-called molecular scattering). In this case, the light is diffracted from random compactions of the medium due to the random thermal motion of molecules. In this case, the scattered light intensity is low and becomes noticeable at a large thickness of the medium. Molecular scattering explains the blue color of the sky and the yellow color of the solar disk. Since the light passing through the atmosphere consists mainly of long waves.

Of the five senses, vision gives us the most information about the world around us. But we can see the world around us only because light enters our eyes. So, we begin the study of light, or optical (Greek optikos - visual), phenomena, that is, phenomena associated with light.

Watching Light Phenomena

We encounter light phenomena every day, because they are part of the natural environment in which we live.

Some optical phenomena seem to us a real miracle, for example, mirages in the desert, auroras. But you must admit that more familiar light phenomena: the sparkle of a dew drop in a sunbeam, a moonlit path on the water, a seven-color rainbow bridge after a summer rain, lightning in thunderclouds, twinkling stars in the night sky are also amazing, because they make the world around us beautiful. full of magical beauty and harmony.

Understanding Light Sources

Light sources are physical bodies whose particles (atoms, molecules, ions) emit light.

Look around, refer to your experience - and you will no doubt name many sources of light: a star, a flash of lightning, a candle flame, a lamp, a computer monitor, etc. (see, for example, Fig. 9.1). Organisms can also emit light: fireflies are bright points of light that can be seen on warm summer nights in forest grass, some marine animals, radiolarians, etc.

On a clear moonlit night, one can see quite well objects illuminated by moonlight. However, the Moon cannot be considered a source of light, because it does not emit, but only reflects the light coming from the Sun.

Is it possible to call a mirror a source of light, with the help of which you start up a "sunbeam"? Explain your answer.

Distinguishing light sources

Rice. 9.2. Powerful sources of artificial light - halogen lamps in the headlights of a modern car

Rice. 9.3. Signals of modern traffic lights are clearly visible even in bright sunshine.

In these traffic lights, incandescent lamps are replaced by LEDs.

Depending on the origin, natural and artificial (man-made) light sources are distinguished.

Natural light sources include the Sun and stars, hot lava and aurora, some living organisms (deep-sea cuttlefish, luminous bacteria, fireflies), etc.

Even in ancient times, people began to create artificial light sources. At first it was bonfires, torches, later - torches, candles, oil and kerosene lamps; at the end of the 19th century. the electric lamp was invented. Today, different types of electric lamps are used everywhere (Fig. 9.2, 9.3).

What types of electric lamps are used in residential buildings? What lamps are used for multi-colored illumination?

There are also thermal and fluorescent light sources.

Heat sources emit light due to the fact that they have a high temperature (Fig. 9.4).

For the glow of luminescent light sources, a high temperature is not needed: the light radiation can be quite intense, while the source remains relatively cold. Examples of fluorescent light sources are aurora and marine plankton, phone screen, fluorescent lamp, fluorescent road sign, etc.

Rice. 9.4. Some thermal light sources


Studying point and extended light sources

A light source that emits light equally in all directions and whose dimensions, given the distance to the observation point, can be neglected, is called a point light source.

A clear example of point sources of light is the stars: we observe them from the Earth, that is, from a distance that is millions of times greater than the size of the stars themselves.

Light sources that are not point-like are called extended light sources. In most cases, we are dealing with extended light sources. This is a fluorescent lamp, and a mobile phone screen, and a candle flame, and a campfire.

Depending on the conditions, the same light source can be considered both extended and point.

On fig. 9.5 shows a lamp for landscape garden lighting. What do you think, in what case can this lamp be considered a point source of light?

We characterize light receivers

Light receivers are devices that change their properties under the influence of light and with the help of which light radiation can be detected.

Light receivers are artificial and natural. In any light receiver, the energy of light radiation is converted into other types of energy - thermal, which manifests itself in the heating of bodies that absorb light, electrical, chemical and even mechanical. As a result of such transformations, the receivers react in a certain way to light or its change.

For example, some security systems operate on photoelectric light receivers - photocells. Beams of light penetrating the space around the protected object are directed to photocells (Fig. 9.6). If one of these beams is blocked, the photocell will not receive light energy and will immediately “report” this.

In solar panels, photovoltaic cells convert light energy into electrical energy. Many modern solar power plants are large "energy fields" of solar panels.

For a long time, only photochemical light detectors (photographic film, photographic paper) were used to take photographs, in which certain chemical reactions occur as a result of the action of light (Fig. 9.7).

From the star closest to us, Alpha Centauri, light travels to Earth for almost 4 years. So, when we look at this star, we actually see what it was like 4 years ago. But there are galaxies that are millions of light years away from us (that is, light travels to them for millions of years!). Imagine that there is a high-tech civilization in such a galaxy. Then it turns out that they see our planet as it was in the time of the dinosaurs!

In modern digital cameras, instead of film, a matrix consisting of a large number of photocells is used. Each of these elements receives "its" part of the light flux, converts it into an electrical signal and transmits this signal to a certain place on the screen.

The natural receivers of light are the eyes of living beings (Fig. 9.8). Under the influence of light, certain chemical reactions occur in the retina of the eye, nerve impulses arise, as a result of which the brain forms an idea of ​​the world around us.

Learn about the speed of light

When you look at the starry sky, you can hardly guess that some stars have already gone out. Moreover, several generations of our ancestors admired the same stars, and these stars did not exist even then! How can it be that there is light from a star, but there is no star itself?

The fact is that light propagates in space at a finite speed. The speed c of light propagation is enormous, and in a vacuum it is about three hundred thousand kilometers per second:

Light travels miles of distance in thousandths of a second. That is why, if the distance from the light source to the receiver is small, it seems that the light propagates instantly. But from distant stars, light travels to us for thousands and millions of years.


Summing up

Physical bodies whose atoms and molecules emit light are called light sources. Light sources are thermal and luminescent; natural and artificial; point and extended. For example, the aurora is a naturally extended luminescent light source.

Devices that change their parameters as a result of the action of light and with the help of which light radiation can be detected are called light receivers. In light receivers, the energy of light radiation is converted into other forms of energy. The organs of vision of living beings are natural receivers of light.

Light propagates in space at a finite speed. Speed

propagation of light in vacuum is approximately: c = 3 10 m/s. Control questions

1. What role does light play in human life? 2. Define a light source. Give examples. 3. Is the moon a source of light? Explain your answer. 4. Give examples of natural and artificial light sources. 5. What do thermal and fluorescent light sources have in common? What is the difference? 6. Under what conditions is a light source considered a point? 7. What devices are called light receivers? Give examples of natural and artificial light receivers. 8. What is the speed of light propagation in vacuum?

Exercise number 9

1. Establish a correspondence between the light source (see figure) and its type.

A Natural thermal B Artificial thermal C Natural luminescent D Artificial luminescent

2. For each line, determine the "extra" word or phrase.

a) candle flame, sun, star, moon, LED lamp;

b) the screen of the switched on computer, lightning, incandescent lamp, torch;

c) fluorescent lamp, gas burner flame, fire, radiolaria.

3. For what approximate time does light travel the distance from the Sun to the Earth - 150 million km?

4. In which of the indicated cases can the Sun be considered a point source of light?

a) observing a solar eclipse;

b) observation of the Sun from a spacecraft flying outside the solar system;

c) determining the time using a sundial.

5. One of the units of length used in astronomy is the light year. How many meters is a light year if it is equal to the distance that light travels in vacuum in one year?

6. Use additional sources of information and find out who and how first measured the speed of light propagation.

This is textbook material.