93. What are called sources of light (§49)?

All bodies from which light emanates are called light sources. There are thermal and luminescent light sources, sources of reflected light:

- thermal light sources they emit light because they have a high temperature (the sun, stars, flame, electric lamp filament); bodies begin to emit light at a temperature of about 800 ° C; invented the electric lamp Alexander Nikolaevich Lodygin (1847-1923, Russia), modern look to the lamp conveyed Thomas Edison (1847-1931, USA);

- luminescent light sources- these are cold light sources, the radiation of which does not depend on temperature (fluorescent and gas-light lamps, TV screen, computer monitor, display of electronic devices, LEDs, rot, fireflies, some marine animals);

- reflected light sources do not radiate themselves; they glow only when light falls on them from some source. For example, the Moon, planets and their satellites, artificial satellites of the Earth reflect the light of the Sun; At night, objects are visible because they reflect moonlight or light from thermal and luminescent sources.

94. How does light propagate in a homogeneous medium (§50)?

In a homogeneous medium consisting of the same substance (for example, air, glass, water), light propagates straightforward.

The rectilinear propagation of light was established by the founder of geometry Euclid (325-265 BC, Dr. Greece).

95. What is a light beam and a light beam (§51)?

- light beam is a narrow limited luminous flux; beams of light can be isolated using small holes in opaque plates called diaphragms.

The beam of light may be parallel(a), divergent(b), converging(in).

Light beams from different sources do not depend on each other and do not affect each other's propagation. This property is called independence of light beams.

- light beam- This is a line indicating the direction of light propagation and is used to represent light beams.

96. What is a point source of light (§52)?

Point light source is a source whose dimensions are small compared to the distance from it to the observer.

97. What is shadow and penumbra (§52).

- Shadow A region of space behind an object that is not exposed to light from a source. The shadow of objects is formed when they are illuminated by point light sources.

- Penumbra is the area that receives light from only part of the light source.

When objects are illuminated by extended light sources, an area is formed shadows and shadows. For example, when the Moon is between the Sun and the Earth, a region of shadow (total solar eclipse) and penumbra (partial solar eclipse) falls from the Moon to the Earth.

98. What is the law of reflection of light (§53)?

Law of light reflection thing is:

The angle of reflection of light is equal to the angle of incidence:

The incident ray, the reflected ray and the perpendicular raised at the point of incidence of the ray to the interface between two media lie in the same plane.

The incident and reflected rays are reversible. For example, if a light beam falls on a mirror in the direction of AO, then it will be reflected in the direction of OB; if the light falls on the mirror in the direction BO, then the beam OA will be reflected.

99. What is specular and diffuse reflection of light (§53)?

- Mirrored Such a reflection is called when a smooth (mirror) surface remains parallel even after reflection. Mirrors reflect smooth polished surfaces, mirrors, water surface.

- diffuse Such a reflection is called when a parallel beam of light incident on a rough surface is reflected diffusely, i.e. rays will be directed in different directions. Thanks to diffuse (scattered) reflection, we see the surrounding objects, the world around us.

100. According to what laws is an object depicted in a flat mirror (§54)?

- flat mirror gives a direct and imaginary image of the object.

The image of an object in a plane mirror has the same dimensions as the object.

The distance from the object to the flat mirror is equal to the distance from the mirror to the image, i.e. the object and its image are symmetrical with respect to the mirror.

The plane mirror gives imaginary(invalid, apparent) image of an object.

101. What spherical mirrors do you know and what parameters are they characterized by (§55)?

- Spherical mirrors are part of the surface of the hollow sphere. Spherical mirrors are concave and convex. For a concave mirror, the inner concave surface of the hollow sphere is mirrored. For a convex mirror, the mirror surface is the outer convex surface of the hollow sphere.

Spherical mirrors are characterized pole, optical center, radius, main optical axis, main focus and focal length.

In the figure: point C is the pole of the mirror; v. O - optical center; СО is the radius of the mirror; direct CO is the main optical axis of the mirror; point F is the main focus of the mirror; distance FC is the focal length of the mirror.

Concave mirrors are used:

When you need to create a parallel beam of light. To do this, a luminous lamp is placed at the focus of the mirror. This is used in flashlights, car headlights, spotlights:

When you need to collect in focus a beam of parallel rays incident on a mirror. This is used in a reflecting telescope.

102. What is called the refraction of light ($57)?

The change in the direction of light propagation when moving from one medium to another is called refraction of light.

103. What characterizes the optical density of a medium (§57)?

Optical density of the medium characterized by the speed of propagation of light in it. The greater the speed of light propagation, the lower the optical density of the medium. For example, the optical density of vacuum, where the speed of light is maximum and is = 300,000 km/s, is equal to 1.

104. How is the law of refraction of light formulated (§57)?

- If a light beam passes from an optically less dense medium to an optically denser one (for example, from air to water), then the angle of refraction is less than the angle of incidence (< ).

If light passes from an optically denser medium to an optically less dense one (for example, from water to air), then the angle of refraction is greater than the angle of incidence ( > ).

The incident and refracted rays, as well as the perpendicular raised at the point of incidence of the ray to the interface between two media, lie in the same plane.

- The sine of the angle of incidence relates to the sine of the angle of refraction as much as the speed of light in the first medium to the speed of light in the second medium: .

105. What is called the limiting angle of total internal reflection (§58)?

Phenomenon total internal reflection observed when a beam of light passes from an optically denser medium to an optically less dense medium. The angle of incidence at which total internal reflection occurs is called limiting angle of total internal reflection.

The phenomenon of total internal reflection is used, for example, in prisms to change the direction of light rays. Such prisms are used in binoculars, periscopes.

106. What is called a light guide and fiber optics (§59)?

Flexible glass rods, in which a light beam entering from one end, repeatedly experiencing total internal reflection, completely exits from the other end, is called a light guide. A new branch of optics based on the use of light guides to transmit information is called fiber optics.

107. What is called a lens? What are the types of lenses (§60)?

Lens called a transparent body bounded by two spherical surfaces. Lenses are convex (collecting) and concave (scattering).

108. What is called the optical center, the main focus and the focal length of the lens (§60)?

- Main optical axis is a line passing through the centers of the spherical surfaces that bound the lens.

- Optical center of the lens is the point through which light rays pass without refraction. Rays pass through the optical center of the lens without refraction.

- Main lens focus- this is the point at which, after refraction, the rays of light falling on the lens parallel to the main optical axis will gather.

109. What is called the optical power of a lens (§60)?

The reciprocal of the focal length is called optical power of the lens: . Optical power is measured in diopters(dptr). 1 diopter = 1/m.

110. How is the lens formula (§61) read?

The sum of the reciprocal distances from the object to the lens and from the lens to the image is equal to the reciprocal of the focal length: .

111. What is the magnification of a lens ($61)?

Lens magnification equals the ratio of the distance from the lens to the image to the distance from the object to the lens: .

112. What parts does the eye consist of ($63)?

Eye a person has a spherical shape with a diameter of 25 cm. Outside it is covered with a strong white shell called sclera (1) . The anterior transparent part of the sclera is called cornea (2) . Behind the cornea is iris (3), determining eye color. In the center of the iris is pupil, followed by a transparent lens (4), shaped like a converging lens. The optical system of the eye gives on its back wall, called retina (5), real, reduced and inverted image of the object.

113. What is called (§63): accommodation of the eye? angle of view? best vision distance?

- accommodation of the eye called the adaptation of the eye to a change in the distance to the object by regulating the curvature of the lens.

- angle of view called the angle at which an object is seen from the optical center of the eye.

- Best vision distance in a normal adult eye is 25 cm, in children - about 10 cm.

114. What is the difference between short-sightedness and far-sightedness (§64)?

There are two main visual impairments: myopia and farsightedness.

A clear image of an object in near-sighted people is obtained in front of the retina, in far-sighted people - behind the retina.

Myopia is corrected by wearing glasses with diverging (concave) lenses, farsightedness - with collecting (convex) lenses.

115. Name optical devices and their purposes (§64).

Optical devices called devices, the action of which is based on the use of lenses. This is:

- glasses used to correct myopia and hyperopia;

- magnifying glass- a lens with a small focal length (from 1 to 10 cm), used to view small objects;

- microscope, designed to examine microscopic bodies;

- binoculars to observe distant bodies;

- telescope for the study of celestial bodies;

- periscope for observation from behind cover;

- camera to obtain clear photographic images of objects;

- projection devices - overhead projector, film projector, graphic projector- designed to obtain an enlarged image of an object on the screen.

116. How is the magnification of a magnifying glass calculated (§64)?

magnifying glass- This is a lens with a small focal length (from 1 to 10 cm), used to view small objects.

Magnifying glass is equal to the ratio of the distance of the best view to the focal length of the magnifying glass: .

117. What is called the spectrum of white color (§65)?

White is complex; it consists of seven simple colors.

The spectrum of white is a multi-colored band obtained as a result of the decomposition of white light and consisting of seven simple colors: red, orange, yellow, green, blue, indigo and violet (every hunter wants to know where the pheasant is sitting).

If a parallel beam of light is directed to a trihedral prism, then a multi-colored band is obtained on the screen, called the white light spectrum. The spectrum arises because beams of different colors are refracted differently by a prism. Red rays are refracted more weakly, while violet rays are more strongly refracted. The rest of the colors are in between.

An example of the spectrum of sunlight is a rainbow, formed by the decomposition of white light on transparent raindrops.

118. What colors are called (§66): complementary? basic?

- Additional The colors that add up to white are called.

- Three spectral colors - red, green and blue - are called primary. Because none of them can be obtained by adding other colors of the spectrum; the addition of these three colors can give white; depending on the proportion in which these colors are added, you can get different colors and shades.

119. Explain the origin (§67): a) the colorlessness of bodies, b) the transparency of bodies, c) the color of the surface of bodies.

Three phenomena occur at the interface between two media: reflection (scattering), refraction, and absorption of light. The color of a body illuminated by white light depends on what color of light this body scatters, transmits or absorbs.

Transparent or colorless bodies (for example, glass, water, air) weakly reflect and let through all colors of white light.

Red glass absorbs all colors except red. Green glass absorbs all colors except green.

The color of a body illuminated by white light is determined by the color it reflects. For example, a red body reflects red and absorbs other colors.

The white body (paper, snow, canvas) reflects all colors.

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 incredible 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.

It's hard to overestimate. All human activity depends on it from the initial periods to the present day. For light fluxes, the Earth's atmosphere, which is in constant motion, is a kind of optical system in which the parameters are constantly changing.

Examples of light phenomena in the atmosphere

The layers of the gaseous shell of our planet are mixed, changing their density, transparency, part of the light is reflected in them, illuminating the earth's surface. In certain cases, the path of the rays is bent, creating the most amazing and colorful phenomena in the atmosphere. Some of them are very common, while others are not well known to people.

Not all physical phenomena are accessible to our eye. The light patterns of a star trail, for example, can only be detected with a long-exposure camera that captures how the stars leave unique trails in the sky as the earth rotates on its axis. Therefore, special optical devices are often used.

Natural atmospheric phenomena, which are the interaction of the play of light and the gaseous shell of our planet, are amazing in beauty and accessible for observation. Most often they arise due to the scattering of rays, their refraction and diffraction when they go around the boundaries of opaque bodies. In the article, we consider unique examples of light phenomena that occur in the atmosphere.

Rainbow

In ancient times, it was considered a bridge connecting earth and sky. The philosopher Descartes substantiated the theory of the appearance of the rainbow, based on the refraction of light rays. However, neither he nor Newton, who supplemented his knowledge, could explain the origin of several such phenomena simultaneously observed in the sky. And only in the 19th century, the astronomer Erey was able to give an explanation for this phenomenon: he considered the curtain of rain as a structure in which light diffraction occurred. His theory is still relevant today. A rainbow is observed when the sun's rays illuminate the veil of rain, located on the side of the sky opposite the luminary. Often, not one, but several rainbows appear to the eyes of the admiring viewer, but the arrangement of colors in them is always the same.

Such light phenomena in living nature are observed not only in the haze of rain, but also on drops of water in fountains, and the moon, the sun and an ordinary searchlight serve as the source of light. Interestingly, scientists who set out to reproduce the phenomenon in artificial conditions received about nineteen images.

Undoubtedly, everyone saw the usual rainbow, but the night one is considered a rare natural phenomenon. In the moonlight, it seems white, but as soon as the raindrops become larger, it immediately turns into color. This phenomenon is still often observed over falling waterfalls.

fiery rainbow

Scientists attribute it to the rarest. It appears with a special arrangement of the sun above the horizon against the background of ice crystals, whose faces are parallel to the earth. Only under such conditions, the light passes into the vertical face, refracts and goes out into the horizontal. And then clouds appear to our astonished eyes, resembling a multi-colored blazing fire, the sky seems to be covered with an iridescent film.

light pole

In ancient times, light phenomena created by the sun were often mistaken for mystical omens. Physics, on the other hand, explains such pillars by the play of sunlight with ice crystals formed in the upper ones. A natural phenomenon will always have the color of the light source, and it can be the sun, moon or any lamp. But if they are formed by natural luminaries, then such columns turn out to be much longer.

Sound and light phenomena accompany the appearance of the aurora, because bright flashes are accompanied by noises and crackles that affect radio transmitters, as a result of which communication is interrupted or completely stopped.

Finally

The physical nature of light phenomena has been the subject of research by people since ancient times. The optical effects that occur in the atmospheric layers of the earth are considered and substantiated from a scientific point of view. The examples of light phenomena in physics given in the review, and not only them, have repeatedly become a real shock for a person, however, even the most complex and bizarre pictures are now finding their explanation. And many phenomena were repeated in artificial conditions. The play of light has long attracted and for a long time will be the subject of admiration for other generations watching how a sunbeam or moonlight gives our planet a unique look.

Atmospheric optical phenomena amaze the imagination with the beauty and variety of illusions created. The most spectacular are pillars of light, false suns, fiery crosses, a gloria and a brocken ghost, which are often mistaken by ignorant people for a Miracle or Epiphany.

Near-horizontal arc, or "fiery rainbow". Light passes through ice crystals in cirrus clouds. A very rare occurrence, since both ice crystals and sunlight must be at a certain angle to each other in order to create the "fire rainbow" effect.

"Ghost of the Brocken". The phenomenon got its name from the top of the Brocken in Germany, where this effect can be regularly observed: a person standing on a hill or mountain, behind whom the sun rises or sets, finds that his shadow, which has fallen on the clouds, becomes incredibly huge. This is due to the fact that the smallest drops of fog refract and reflect sunlight in a special way.

circumzenithal arc. An arc centered at the zenith, located approximately 46° above the Sun. It is rarely visible and only for a few minutes, has bright colors, clear outlines and is always parallel to the horizon. To an outside observer, she will remind the smile of the Cheshire Cat or an inverted rainbow.

"Foggy" rainbow. The misty halo looks like a colorless rainbow. The fog that gives rise to this halo consists of smaller particles of water, and the light, refracted in tiny droplets, does not color it.

Gloria. This effect can only be observed on clouds that are directly in front of the viewer or below him, at a point that is on the opposite side to the light source. Thus, Gloria can only be seen from a mountain or from an airplane, and the light sources (the Sun or the Moon) should be directly behind the observer.

Halo at 22º. The white circles of light around the Sun or Moon, which result from the refraction or reflection of light by ice or snow crystals in the atmosphere, are called halos. During the cold season, halos formed by ice and snow crystals on the earth's surface reflect sunlight and scatter it in different directions, forming an effect called "diamond dust".

Rainbow clouds. When the Sun is at a certain angle to the water droplets that make up the cloud, these droplets refract the sunlight and create an unusual "rainbow cloud" effect, coloring it in all the colors of the rainbow.

Lunar rainbow (night rainbow)- a rainbow generated by the moon to a greater extent than the sun. The lunar rainbow is comparatively paler than the usual one. This is because the moon produces less light than the sun. The lunar rainbow is always on the opposite side of the sky from the moon.

Parhelion- one of the halo forms, in which one or more additional images of the Sun are observed in the sky.
The Tale of Igor's Campaign mentions that before the offensive of the Polovtsians and the capture of Igor, "four suns shone over the Russian land." The warriors took this as a sign of impending big trouble.

Aurora borealis- the glow of the upper layers of the atmospheres of planets with a magnetosphere due to their interaction with charged particles of the solar wind.

Saint Elmo's fire- a discharge in the form of luminous beams or tassels that occur at the sharp ends of tall objects (towers, masts, lonely trees, sharp rock tops, etc.) at high electric field strength in the atmosphere.

Zodiacal light. The diffuse glow of the night sky, created by sunlight reflected from interplanetary dust particles, is also called zodiacal light. The zodiacal light can be observed in the evening in the west or in the morning in the east.

Pillars of light. Flat ice crystals reflect light in the upper atmosphere and form vertical pillars of light that seem to emerge from the earth's surface. Light sources can be the Moon, the Sun or lights of artificial origin.

Star track. Invisible to the naked eye, it can be captured on a camera.

White rainbow. Photo taken on the Golden Gate Bridge in San Francisco

Buddha Light. The phenomenon is similar to the Brocken Ghost. The sun's rays are reflected from the atmospheric water droplets over the sea and the shadow of the plane in the middle of the rainbow circle...

Green beam."When the setting Sun is completely out of sight, the last glimpse looks strikingly green. The effect can only be seen from places where the horizon is low and far away. It lasts only a few seconds."

Mirage, a well-known natural phenomenon...

moon rainbow- this is a rather rare phenomenon in the Earth's atmosphere and appears only when the moon is full. For the appearance of a lunar rainbow, it is necessary: ​​the full moon, not covered by clouds, and the fall of heavy rain. A real lunar rainbow is half the size of the sky.

mountain shadow, observed against the background of evening clouds:

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 types 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. test 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.