The blue color of the sky is due to the fact that... What color is the sky? Why is the sky blue from a physics point of view? Hypotheses put forward at different times

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1. Introduction.

While playing on the street, I once noticed the sky, it was extraordinary: bottomless, endless and blue, blue! And only the clouds slightly covered this blue color. I wondered, why is the sky blue? I immediately remembered the song of the fox Alice from the fairy tale about Pinocchio “What a blue sky...!” and a geography lesson, where, while studying the topic “Weather,” we described the state of the sky, and also said that it was blue. So after all, why is the sky blue? When I got home, I asked my mother this question. She told me that when people cry, they ask heaven for help. The sky takes away their tears, so it turns blue like a lake. But my mother’s story did not satisfy my question. I decided to ask my classmates and teachers if they knew why the sky was blue? 24 students and 17 teachers took part in the survey. After processing the questionnaires, we received the following results:

At school, during a geography lesson, I asked the teacher this question. She answered me that the color of the sky can be easily explained from the point of view of physics. This phenomenon is called dispersion. From Wikipedia I learned that dispersion is the process of decomposing light into a spectrum. Geography teacher Larisa Borisovna suggested that I observe this phenomenon experimentally. And we went to the physics room. Vasily Aleksandrovich, a physics teacher, willingly agreed to help us with this. Using special equipment, I was able to trace how the dispersion process occurs in nature.

In order to find the answer to the question why the sky is blue, we decided to conduct a study. This is how the idea of writing a project came about. Together with my supervisor, we determined the topic, purpose and objectives of the research, put forward a hypothesis, determined research methods and mechanisms for implementing our idea.

Hypothesis: Light is sent to the Earth by the Sun and most often when we look at it, it appears dazzlingly white to us. Does that mean the sky should be white? But in reality the sky is blue. In the course of the study we will find explanations for these contradictions.

Target: find the answer to the question why the sky is blue and find out what its color depends on.

Tasks: 1. Familiarize yourself with theoretical material on the topic

2. Experimentally study the phenomenon of light dispersion

3. Observe the color of the sky at different times of the day and in different weather conditions

Object of study: sky

Item: light and color of the sky

Research methods: analysis, experiment, observation

Stages of work:

1. Theoretical

2. Practical

3. Final: conclusions on the research topic

Practical significance of the work: Research materials can be used in geography and physics lessons as a teaching module.

2. Main part.

2.1. Theoretical aspects of the problem. The phenomenon of blue sky from the point of view of physics

Why is the sky blue - it is very difficult to find an answer to such a simple question. First, let's define the concept. The sky is the space above the Earth or the surface of any other astronomical object. In general, the sky is usually called the panorama that opens when looking from the surface of the Earth (or other astronomical object) towards space.

Many scientists have racked their brains in search of an answer. Leonardo da Vinci, watching the fire in the fireplace, wrote: “Light over darkness becomes blue.” But today it is known that the fusion of white and black produces gray.

Rice. 1. Leonardo da Vinci's hypothesis

Isaac Newton almost explained the color of the sky, however, for this he had to assume that the drops of water contained in the atmosphere have thin walls like soap bubbles. But it turned out that these drops are spheres, which means they have no wall thickness. And so Newton's bubble burst!

Rice. 2. Newton's hypothesis

The best solution to the problem was proposed by the English physicist Lord John Rayleigh about 100 years ago. But let's start from the beginning. The sun emits a blinding white light, which means the color of the sky should be the same, but it is still blue. What happens to white light in the atmosphere? When passing through the atmosphere, as if through a prism, it breaks up into seven colors. You probably know these lines: every hunter wants to know where the pheasant sits. There is a deep meaning hidden in these sentences. They represent to us the primary colors in the visible light spectrum.

Rice. 3. Spectrum of white light.

The best natural demonstration of this spectrum is, of course, the rainbow.

Rice. 4 Visible light spectrum

Visible light is electromagnetic radiation whose waves have different wavelengths. There is also invisible light; our eyes do not perceive it. These are ultraviolet and infrared. We don't see it because its length is either too long or too short. Seeing light means perceiving its color, but what color we see depends on the wavelength. The longest visible waves are red, and the shortest are violet.

The ability of light to scatter, that is, to propagate in a medium, also depends on the wavelength. Red light waves scatter the worst, but blue and violet colors have a high scattering ability.

Rice. 5. Light scattering ability

And finally, we are close to the answer to our question, why is the sky blue? As mentioned above, white is a mixture of all possible colors. When it collides with a gas molecule, each of the seven color components of white light is scattered. At the same time, light with longer waves is scattered worse than light with short waves. Because of this, 8 times more blue spectrum remains in the air than red. Although violet has the shortest wavelength, the sky still appears blue due to the mixture of violet and green wavelengths. In addition, our eyes perceive blue better than violet, given the same brightness of both. It is these facts that determine the color scheme of the sky: the atmosphere is literally filled with rays of blue-blue color.

However, the sky is not always blue. During the day we see the sky as blue, cyan, gray, in the evening - red (Annex 1). Why is the sunset red? During sunset, the Sun approaches the horizon, and the sun's ray is directed towards the Earth's surface not vertically, as during the day, but at an angle. Therefore, the path it takes through the atmosphere is much longer than what it takes during the day when the Sun is high. Because of this, the blue-blue spectrum is absorbed in the atmosphere before reaching the Earth, and longer light waves of the red spectrum reach the Earth's surface, coloring the sky in red and yellow tones. The change in color of the sky is clearly related to the rotation of the Earth around its axis, and therefore the angle of incidence of light on the Earth.

2.2. Practical aspects. Experimental way to solve the problem

In the physics class I became acquainted with the spectrograph device. Vasily Aleksandrovich, a physics teacher, told me the operating principle of this device, after which I independently conducted an experiment called dispersion. A ray of white light passing through a prism is refracted and we see a rainbow on the screen. (Appendix 2). This experience helped me understand how this amazing creation of nature appears in the sky. With the help of a spectrograph, scientists today can obtain information about the composition and properties of various substances.

Photo 1. Demonstration of dispersion experience in

physics room

I wanted to get a rainbow at home. My geography teacher, Larisa Borisovna, told me how to do this. An analogue of the spectrograph was a glass container with water, a mirror, a flashlight and a white sheet of paper. Place a mirror in a container of water and place a white sheet of paper behind the container. We direct the light of a flashlight onto the mirror so that the reflected light falls on the paper. A rainbow has appeared on a piece of paper again! (Appendix 3). It is better to conduct the experiment in a darkened room.

We have already said above that white light essentially already contains all the colors of the rainbow. You can make sure of this and collect all the colors back to white by making a rainbow top (Appendix 4). If you spin it too much, the colors will merge and the disc will turn white.

Despite the scientific explanation for the formation of a rainbow, this phenomenon remains one of the mysterious optical spectacles in the atmosphere. Watch and enjoy!

3. Conclusion

In search of an answer to the children’s question so often asked by parents, “Why is the sky blue?” I learned a lot of interesting and instructive things. The contradictions in our hypothesis today have a scientific explanation:

The whole secret is in the color of the sky in our atmosphere - in the air envelope of planet Earth.

A white ray of sun, passing through the atmosphere, breaks up into rays of seven colors.

Red and orange rays are the longest, and blue rays are the shortest.

Blue rays reach the Earth less than others, and thanks to these rays the sky is permeated with blue color

The sky is not always blue and this is due to the axial movement of the Earth.

Experimentally, we were able to visualize and understand how dispersion occurs in nature. During homework at school, I told my classmates why the sky is blue. It was also interesting to know where the phenomenon of dispersion can be observed in our daily lives. I have found several practical uses for this unique phenomenon. (Appendix 5). In the future I would like to continue studying the sky. How many more mysteries does it hold? What other phenomena occur in the atmosphere and what is their nature? How do they affect humans and all life on Earth? Perhaps these will be the topics of my future research.

Bibliography

1. Wikipedia - the free encyclopedia

2. L.A. Malikova. Electronic textbook on physics “Geometric optics”

3. Peryshkin A.V. Physics. 9th grade. Textbook. M.: Bustard, 2014, p.202-209

4. htt;/www. voprosy-kak-ipochemu.ru

5. Personal photo archive “Sky over Golyshmanovo”

Annex 1.

"The sky above Golyshmanovo"(personal photo archive)

Appendix 2.

Dispersion of light using a spectrograph

Appendix 3.

Light dispersion at home

"rainbow"

Appendix 4.

Rainbow top

Top at rest Top during rotation

Appendix 5.

Variation in human life

Diamond Lights on board an airplane

Car headlights

Reflective signs

On a clear sunny day, the sky above us looks bright blue. In the evening, the sunset colors the sky in red, pink and orange. Why is the sky blue? What makes a sunset red?

To answer these questions, you need to know what light is and what the Earth's atmosphere is made of.

Atmosphere

The atmosphere is a mixture of gases and other particles that surround the earth. The atmosphere mainly consists of nitrogen (78%) and oxygen (21%) gases. Argon gas and water (in the form of steam, droplets and ice crystals) are the next most common in the atmosphere, their concentration does not exceed 0.93% and 0.001%, respectively. The Earth's atmosphere also contains small quantities of other gases, as well as tiny particles of dust, soot, ash, pollen and salt that enter the atmosphere from the oceans.

The composition of the atmosphere varies within small limits depending on location, weather, etc. The concentration of water in the atmosphere increases during storms, as well as near the ocean. Volcanoes are capable of throwing huge amounts of ash high into the atmosphere. Man-made pollution can also add various gases or dust and soot to the normal composition of the atmosphere.

The density of the atmosphere at low altitudes near the Earth's surface is greatest; with increasing altitude it gradually decreases. There is no clearly defined boundary between the atmosphere and space.

Light waves

Light is a type of energy that is transported by waves. In addition to light, waves carry other types of energy, for example, a sound wave is a vibration of air. A light wave is an oscillation of electric and magnetic fields, this range is called the electromagnetic spectrum.

Electromagnetic waves travel through airless space at a speed of 299.792 km/s. The speed at which these waves propagate is called the speed of light.

Radiation energy depends on the wavelength and its frequency. Wavelength is the distance between the two closest peaks (or troughs) of the wave. The frequency of a wave is the number of times a wave oscillates per second. The longer the wave, the lower its frequency, and the less energy it carries.

Visible light colors

Visible light is the part of the electromagnetic spectrum that can be seen by our eyes. The light emitted by the Sun or an incandescent lamp may appear white, but it is actually a mixture of different colors. You can see the different colors of the visible spectrum of light by breaking it down into its components using a prism. This spectrum can also be observed in the sky in the form of a rainbow, resulting from the refraction of light from the Sun in droplets of water, acting as one giant prism.

The colors of the spectrum mix and continuously transform into one another. At one end the spectrum has red or orange colors. These colors smoothly transition into yellow, green, blue, indigo and violet. Colors have different wavelengths, different frequencies, and differ in energies.

Propagation of light in air

Light travels through space in a straight line as long as there are no obstacles in its path. When a light wave enters the atmosphere, the light continues to travel in a straight line until dust or gas molecules get in its way. In this case, what happens to the light will depend on its wavelength and the size of the particles caught in its path.

Dust particles and water droplets are much larger than the wavelength of visible light. Light is reflected in different directions when it hits these large particles. Different colors of visible light are reflected equally by these particles. Reflected light appears white because it still contains the same colors that were present before it was reflected.

Gas molecules are smaller than the wavelength of visible light. If a light wave collides with them, the result of the collision may be different. When light collides with a molecule of any gas, some of it is absorbed. A little later, the molecule begins to emit light in different directions. The color of the light emitted is the same color that was absorbed. But colors of different wavelengths are absorbed differently. All colors can be absorbed, but higher frequencies (blue) are absorbed much more strongly than lower frequencies (red). This process is called Rayleigh scattering, named after the British physicist John Rayleigh, who discovered this scattering phenomenon in the 1870s.

Why is the sky blue?

The sky is blue due to Rayleigh scattering. As light travels through the atmosphere, most of the long wavelengths of the optical spectrum pass through unchanged. Only a small portion of red, orange and yellow colors interact with air.

However, many shorter wavelengths of light are absorbed by gas molecules. Once absorbed, the blue color is emitted in all directions. It is scattered everywhere in the sky. No matter which direction you look, some of this scattered blue light reaches the observer. Since blue light is visible everywhere overhead, the sky appears blue.

If you look towards the horizon, the sky will have a paler hue. This is the result of light traveling a greater distance through the atmosphere to reach the observer. The scattered light is scattered again by the atmosphere and less blue light reaches the observer's eyes. Therefore, the color of the sky near the horizon appears paler or even appears completely white.

Black sky and white sun

From Earth, the Sun appears yellow. If we were in space or on the Moon, the Sun would appear white to us. There is no atmosphere in space to scatter sunlight. On Earth, some of the short wavelengths of sunlight (blue and violet) are absorbed by scattering. The rest of the spectrum appears yellow.

Also, in space, the sky appears dark or black instead of blue. This is the result of the absence of an atmosphere, therefore the light is not scattered in any way.

Why is the sunset red?

When the Sun goes down, sunlight has to travel a greater distance in the atmosphere to reach the observer, so more sunlight is reflected and scattered by the atmosphere. Since less direct light reaches the observer, the Sun appears less bright. The color of the Sun also appears different, ranging from orange to red. This happens because even more short-wavelength colors, blue and green, are scattered. Only the long-wave components of the optical spectrum remain, which reach the observer’s eyes.

The sky around the setting sun can have different colors. The sky is most beautiful when the air contains many small particles of dust or water. These particles reflect light in all directions. In this case, shorter light waves are scattered. The observer sees light rays of longer wavelengths, which is why the sky appears red, pink or orange.

More about the atmosphere

What is atmosphere?

The atmosphere is a mixture of gases and other substances that surround the Earth in the form of a thin, mostly transparent shell. The atmosphere is held in place by the Earth's gravity. The main components of the atmosphere are nitrogen (78.09%), oxygen (20.95%), argon (0.93%) and carbon dioxide (0.03%). The atmosphere also contains small amounts of water (in different places its concentration ranges from 0% to 4%), solid particles, gases neon, helium, methane, hydrogen, krypton, ozone and xenon. The science that studies the atmosphere is called meteorology.

Life on Earth would not be possible without the presence of an atmosphere, which supplies the oxygen we need to breathe. In addition, the atmosphere performs another important function - it equalizes the temperature throughout the planet. If there were no atmosphere, then in some places on the planet there could be sizzling heat, and in other places extreme cold, the temperature range could fluctuate from -170°C at night to +120°C during the day. The atmosphere also protects us from harmful radiation from the Sun and space, absorbing and dispersing it.

Of the total amount of solar energy reaching the Earth, approximately 30% is reflected by clouds and the earth's surface back into space. The atmosphere absorbs approximately 19% of the sun's radiation, and only 51% is absorbed by the Earth's surface.

Air has weight, although we are not aware of it and do not feel the pressure of the air column. At sea level, this pressure is one atmosphere, or 760 mmHg (1013 millibars or 101.3 kPa). As altitude increases, atmospheric pressure decreases rapidly. The pressure drops 10 times with every 16 km increase in altitude. This means that at a pressure of 1 atmosphere at sea level, at an altitude of 16 km the pressure will be 0.1 atm, and at an altitude of 32 km - 0.01 atm.

The density of the atmosphere in its lowest layers is 1.2 kg/m3. Each cubic centimeter of air contains approximately 2.7 * 10 19 molecules. At ground level, each molecule moves at about 1,600 km/h, colliding with other molecules 5 billion times per second.

Air density also decreases rapidly with increasing altitude. At an altitude of 3 km, air density decreases by 30%. People living near sea level experience temporary breathing problems when raised to such a height. The highest altitude at which people permanently live is 4 km.

The structure of the atmosphere

The atmosphere consists of different layers, the division into these layers occurs according to their temperature, molecular composition and electrical properties. These layers do not have clearly defined boundaries; they change seasonally, and in addition, their parameters change at different latitudes.

Division of the atmosphere into layers depending on their molecular composition

Homosphere

The lower 100 km, including the Troposphere, Stratosphere and Mesopause.
Makes up 99% of the mass of the atmosphere.
Molecules are not separated by molecular weight.
The composition is fairly homogeneous, with the exception of some small local anomalies. Homogeneity is maintained by constant mixing, turbulence and turbulent diffusion.
Water is one of two components that are unevenly distributed. As water vapor rises, it cools and condenses, then returning to the ground in the form of precipitation - snow and rain. The stratosphere itself is very dry.
Ozone is another molecule whose distribution is uneven. (Read below about the ozone layer in the stratosphere.)

Heterosphere

Extends above the homosphere and includes the Thermosphere and Exosphere.
The separation of molecules in this layer is based on their molecular weights. Heavier molecules such as nitrogen and oxygen are concentrated at the bottom of the layer. Lighter ones, helium and hydrogen, predominate in the upper part of the heterosphere.

Division of the atmosphere into layers depending on their electrical properties.

Neutral atmosphere

Below 100 km.

Ionosphere

Approximately above 100 km.
Contains electrically charged particles (ions) produced by absorption of ultraviolet light
The degree of ionization changes with altitude.
Different layers reflect long and short radio waves. This allows radio signals traveling in a straight line to bend around the spherical surface of the earth.
Auroras occur in these atmospheric layers.
Magnetosphere is the upper part of the ionosphere, extending to approximately 70,000 km altitude, this altitude depends on the intensity of the solar wind. The magnetosphere protects us from high-energy charged particles from the solar wind by keeping them in the Earth's magnetic field.

Division of the atmosphere into layers depending on their temperatures

Top border height troposphere depends on seasons and latitude. It extends from the earth's surface to an altitude of approximately 16 km at the equator, and to an altitude of 9 km at the North and South Poles.

The prefix "tropo" means change. Changes in the parameters of the troposphere occur due to weather conditions - for example, due to the movement of atmospheric fronts.
As altitude increases, the temperature drops. Warm air rises, then cools and falls back to Earth. This process is called convection, it occurs as a result of the movement of air masses. Winds in this layer blow predominantly vertically.
This layer contains more molecules than all other layers combined.

Stratosphere- extends from approximately 11 km to 50 km altitude.

Has a very thin layer of air.
The prefix "strato" refers to layers or division into layers.
The lower part of the Stratosphere is quite calm. Jet aircraft often fly into the lower stratosphere to avoid bad weather in the troposphere.
At the top of the Stratosphere there are strong winds known as high-altitude jet streams. They blow horizontally at speeds of up to 480 km/h.
The stratosphere contains the "ozone layer", located at an altitude of approximately 12 to 50 km (depending on latitude). Although the concentration of ozone in this layer is only 8 ml/m 3, it is very effective at absorbing harmful ultraviolet rays from the sun, thereby protecting life on earth. The ozone molecule consists of three oxygen atoms. The oxygen molecules we breathe contain two oxygen atoms.
The stratosphere is very cold, with a temperature of approximately -55°C at the bottom and increasing with altitude. The increase in temperature is due to the absorption of ultraviolet rays by oxygen and ozone.

Mesosphere- extends to altitudes of approximately 100 km.

As altitude increases, the temperature rises rapidly.

Thermosphere- extends to altitudes of approximately 400 km.

As altitude increases, the temperature increases rapidly due to the absorption of very short wavelength ultraviolet radiation.
Meteors, or "shooting stars", begin to burn up at altitudes of approximately 110-130 km above the Earth's surface.

Exosphere- extends for hundreds of kilometers beyond the Thermosphere, gradually moving into outer space.

The air density here is so low that the use of the concept of temperature loses all meaning.
When molecules collide with each other, they often fly off into space.

Why is the color of the sky blue?

Visible light is a type of energy that can travel through space. Light from the Sun or an incandescent lamp appears white, although in reality it is a mixture of all colors. The primary colors that make up white are red, orange, yellow, green, blue, indigo and violet. These colors continuously transform into one another, so in addition to the primary colors there are also a huge number of various shades. All these colors and shades can be observed in the sky in the form of a rainbow that appears in an area of high humidity.

The air that fills the entire sky is a mixture of tiny gas molecules and small solid particles such as dust.

As sunlight passes through the air, it encounters molecules and dust. When light collides with gas molecules, light can be reflected in various directions. Some colors, such as red and orange, reach the observer directly by passing directly through the air. But most blue light is reflected from air molecules in all directions. This scatters blue light throughout the sky and makes it appear blue.

When we look up, some of this blue light reaches our eyes from all over the sky. Since we see blue everywhere above our heads, the sky looks blue.

There is no air in outer space. Since there are no obstacles from which the light could be reflected, the light travels directly. The rays of light are not scattered, and the “sky” appears dark and black.

Experiments with light

The first experiment is the decomposition of light into a spectrum

To conduct this experiment you will need:

a small mirror, a piece of white paper or cardboard, water;
a large shallow vessel such as a cuvette or bowl, or a plastic ice cream box;
sunny weather and a window facing the sunny side.

How to conduct an experiment:

Fill the cuvette or bowl 2/3 full with water and place it on the floor or table so that direct sunlight reaches the water. The presence of direct sunlight is mandatory for proper experimentation.
Place the mirror underwater so that the sun's rays fall on it. Hold a piece of paper over the mirror so that the rays of the sun reflected by the mirror fall on the paper; if necessary, adjust their relative position. Observe the color spectrum on paper.

What happens: The water and mirror act like a prism, splitting light into the color components of the spectrum. This happens because light rays, passing from one medium (air) to another (water), change their speed and direction. This phenomenon is called refraction. Different colors are refracted differently, violet rays are more inhibited and change their direction more strongly. Red rays slow down and change direction less. Light is separated into its component colors and we can see the spectrum.

Second experiment - modeling the sky in a glass jar

Materials required for the experiment:

a transparent tall glass or a transparent plastic or glass jar;
water, milk, teaspoon, flashlight;
a dark room;

Conducting the experiment:

Fill a glass or jar 2/3 full with water, approximately 300-400 ml.
Add 0.5 to one spoon of milk to the water, shake the mixture.
Taking a glass and a flashlight, go into a dark room.
Hold a flashlight over a glass of water and direct the light beam at the surface of the water, look at the glass from the side. In this case, the water will have a bluish tint. Now point the flashlight at the side of the glass, and look at the beam of light from the other side of the glass, so that the light passes through the water. In this case, the water will have a reddish tint. Place a flashlight under the glass and direct the light upward, while looking at the water from above. In this case, the reddish tint of the water will look more saturated.

What happens in this experiment is that small particles of milk suspended in water scatter the light coming from a flashlight in the same way that particles and molecules in the air scatter sunlight. When a glass is illuminated from above, the water appears bluish due to the fact that the blue color is scattered in all directions. When you look directly at the light through the water, the light from the lantern appears red because some of the blue rays have been removed due to light scattering.

Third experiment - mixing colors

You will need:

pencil, scissors, white cardboard or piece of whatman paper;
colored pencils or markers, ruler;
a mug or large cup with a diameter at the top of 7...10 cm or a caliper.
Paper cup.

How to conduct an experiment:

If you don't have a caliper, use a mug as a template to draw a circle on a piece of cardboard and cut out the circle. Using a ruler, divide the circle into 7 approximately equal sectors.
Color these seven sectors in the colors of the main spectrum - red, orange, yellow, green, blue, indigo and violet. Try to paint the disc as neatly and evenly as possible.
Make a hole in the middle of the disk and place the disk on a pencil.
Make a hole in the bottom of the paper cup, the diameter of the hole should be slightly larger than the diameter of the pencil. Turn the cup upside down and insert a pencil with a mounted disk into it so that the pencil lead rests on the table, adjust the position of the disk on the pencil so that the disk does not touch the bottom of the cup and is above it at a height of 0.5..1.5 cm.
Quickly spin the pencil and look at the rotating disk, pay attention to its color. If necessary, adjust the disk and pencil so that they can rotate easily.

Explanation of the phenomenon seen: the colors with which the sectors on the disk are painted are the main components of the colors of white light. When the disk spins fast enough, the colors seem to merge and the disk appears white. Try experimenting with other color combinations.

We are all accustomed to the fact that the color of the sky is a variable characteristic. Fog, clouds, time of day - everything affects the color of the dome overhead. Its daily shift does not occupy the minds of most adults, which cannot be said about children. They are constantly wondering why the sky is physically blue or what makes a sunset red. Let's try to understand these not so simple questions.

Changeable

It’s worth starting by answering the question of what the sky actually represents. In the ancient world, it was truly seen as a dome covering the Earth. Today, however, hardly anyone does not know that, no matter how high the curious explorer rises, he will not be able to reach this dome. The sky is not a thing, but rather a panorama that opens up when viewed from the surface of the planet, a kind of appearance woven from light. Moreover, if observed from different points, it may look different. So, from rising above the clouds, a completely different view opens up than from the ground at this time.

A clear sky is blue, but as soon as clouds come in, it becomes gray, leaden or dirty white. The night sky is black, sometimes you can see reddish areas on it. This is the reflection of the artificial lighting of the city. The reason for all such changes is light and its interaction with air and particles of various substances in it.

The nature of color

In order to answer the question of why the sky is blue from a physics point of view, we need to remember what color is. This is a wave of a certain length. Light coming from the Sun to the Earth is seen as white. It has been known since Newton's experiments that it is a beam of seven rays: red, orange, yellow, green, blue, indigo and violet. Colors differ in wavelength. The red-orange spectrum includes waves that are the most impressive in this parameter. parts of the spectrum are characterized by short wavelengths. The decomposition of light into a spectrum occurs when it collides with molecules of various substances, and some of the waves can be absorbed, and some can be scattered.

Investigation of the cause

Many scientists have tried to explain why the sky is blue in terms of physics. All researchers sought to discover a phenomenon or process that scatters light in the planet's atmosphere in such a way that, as a result, only blue light reaches us. The first candidates for the role of such particles were water. It was believed that they absorb red light and transmit blue light, and as a result we see a blue sky. Subsequent calculations, however, showed that the amount of ozone, ice crystals and water vapor molecules in the atmosphere is not enough to give the sky a blue color.

The reason is pollution

At the next stage of research, John Tyndall suggested that dust plays the role of the desired particles. Blue light has the greatest resistance to scattering, and therefore is able to pass through all layers of dust and other suspended particles. Tindall conducted an experiment that confirmed his assumption. He created a smog model in the laboratory and illuminated it with bright white light. The smog took on a blue tint. The scientist made an unambiguous conclusion from his research: the color of the sky is determined by dust particles, that is, if the Earth’s air was clean, then the skies above people’s heads would glow not blue, but white.

Lord's Research

The final point on the question of why the sky is blue (from the point of view of physics) was put by the English scientist, Lord D. Rayleigh. He proved that it is not dust or smog that colors the space above our heads in the shade we are familiar with. It's in the air itself. Gas molecules absorb most and primarily the longest wavelengths, equivalent to red. The blue dissipates. This is precisely how today we explain the color of the sky we see in clear weather.

Those attentive will notice that, following the logic of scientists, the dome overhead should be purple, since this color has the shortest wavelength in the visible range. However, this is not a mistake: the proportion of violet in the spectrum is much smaller than blue, and human eyes are more sensitive to the latter. In fact, the blue we see is the result of mixing blue with violet and some other colors.

Sunsets and clouds

Everyone knows that at different times of the day you can see different colors of the sky. Photos of beautiful sunsets over the sea or lake are a perfect illustration of this. All kinds of shades of red and yellow combined with blue and dark blue make such a spectacle unforgettable. And it is explained by the same scattering of light. The fact is that during sunset and dawn, the sun's rays have to travel a much longer path through the atmosphere than at the height of the day. In this case, the light from the blue-green part of the spectrum is scattered in different directions and clouds located near the horizon become colored in shades of red.

When the sky becomes clouded, the picture changes completely. unable to overcome the dense layer, and most of them simply do not reach the ground. The rays that managed to pass through the clouds meet with water drops of rain and clouds, which again distort the light. As a result of all these transformations, white light reaches the earth if the clouds are small in size, and gray light when the sky is covered by impressive clouds that absorb part of the rays for the second time.

Other skies

It is interesting that on other planets of the solar system, when viewed from the surface, you can see a sky that is very different from the one on Earth. On space objects deprived of an atmosphere, the sun's rays freely reach the surface. As a result, the sky here is black, without any shade. This picture can be seen on the Moon, Mercury and Pluto.

The Martian sky has a red-orange hue. The reason for this lies in the dust that fills the planet’s atmosphere. It is painted in different shades of red and orange. When the Sun rises above the horizon, the Martian sky turns pinkish-red, while the area immediately surrounding the disk of the luminary appears blue or even violet.

The sky above Saturn is the same color as on Earth. Aquamarine skies stretch over Uranus. The reason lies in the methane haze located in the upper planets.

Venus is hidden from the eyes of researchers by a dense layer of clouds. It does not allow rays of the blue-green spectrum to reach the surface of the planet, so the sky here is yellow-orange with a gray stripe along the horizon.

Exploring the space overhead during the day reveals no less wonders than studying the starry sky. Understanding the processes occurring in the clouds and behind them helps to understand the reason for things that are quite familiar to the average person, which, however, not everyone can explain right away.

In short, then ... "Sunlight, interacting with air molecules, is scattered into different colors. Of all the colors, blue is the color that is best prone to scattering. It turns out that he is actually capturing airspace.”

Now let's take a closer look

Only children can ask such simple questions that a completely adult person does not know how to answer. The most common question tormenting children's heads is: “Why is the sky blue?” However, not every parent knows the correct answer even for themselves. The science of physics and scientists who have been trying to answer it for hundreds of years will help you find it.

Erroneous explanations

People have been looking for the answer to this question for many centuries. Ancient people believed that this color was the favorite color of Zeus and Jupiter. At one time, explanations of the color of the sky worried such great minds as Leonardo da Vinci and Newton. Leonardo da Vinci believed that when darkness and light combine with each other, they form a lighter shade - blue. Newton associated the blue color with the accumulation of a large number of water drops in the sky. However, it was only in the 19th century that the correct conclusion was reached.

Range

In order for a child to understand the correct explanation using the science of physics, he first needs to understand that a beam of light is particles flying at high speed - segments of an electromagnetic wave. In a stream of light, long and short rays move together, and are perceived jointly by the human eye as white light. Penetrating into the atmosphere through tiny drops of water and dust, they are scattered into all the colors of the spectrum (rainbow).

John William Rayleigh

Back in 1871, British physicist Lord Rayleigh noticed the dependence of the intensity of scattered light on wavelength. The scattering of light from the Sun by irregularities in the atmosphere explains why the sky is blue. According to Rayleigh's law, blue solar rays are scattered much more intensely than orange and red ones, since they have a shorter wavelength.

The air near the Earth's surface and high in the sky is made up of molecules, causing sunlight to be scattered further up in the air. It reaches the observer from all sides, even from the most distant. The spectrum of air-scattered light is noticeably different from direct sunlight. The energy of the first is shifted to the yellow-green part, and the second - to the blue.

The more intense the direct sunlight scatters, the cooler the color will appear. The strongest dispersion, i.e. The shortest wavelength is in violet, and the longest wavelength is in red. Therefore, during sunset, areas of the sky distant from it appear blue, and those closest to it appear pink or scarlet.

Sunrises and sunsets

During sunset and dawn, people most often see pink and orange hues in the sky. This is because light from the Sun travels very low to the surface of the earth. Because of this, the path that light needs to travel during sunset and dawn is much longer than during the day. Because the rays have the longest path through the atmosphere, most of the blue light is scattered, so the light from the sun and nearby clouds appears reddish or pink to humans.

Probably everyone has faced this simple question at least once in their life: Why is a clear, cloudless sky blue or blue? Obviously, because of the air we breathe, because of the Earth's atmosphere! Our air is probably “blue” or something like that. It only seems transparent, but at great distances planes, mountains, ships seem to be in a bluish haze... Such reasoning does not solve the main question: why is the sky blue? The air is not painted blue!

The simple and short answer is: the sky is blue because air molecules scatter the blue color of the sun more than the red.

Because air scatters blue light, the sky appears blue and the Sun itself appears yellow. Moreover, at sunset, when sunlight passes through a larger thickness of the atmosphere, we see Red sun and the dawn, painted in yellow-red colors. All this is possible only because blue light is scattered by the atmosphere on its way to us.

But where did blue light come from in the first place? To begin with, white light from the Sun is a mixture of all the colors of the rainbow, from violet to red. Stop, you say Is the sun's light white? Yes, . Second point: we are now talking about light, not about color. If we mix paints of different colors, we, of course, get something almost black.

The color of light is not the color of any object. If we mix red, yellow, orange, green, cyan, indigo and violet light in roughly equal amounts, we get white light. Isaac Newton was the first to demonstrate this by using a prism to separate different colors and form a spectrum.

Scientists have discovered that colored light is just light of different wavelengths. The visible part of the spectrum ranges from red light with a wavelength of about 720 nm to violet with a wavelength of about 380 nm, with orange, yellow, green, cyan and blue in between. Three different types of color receptors in the retina of the human eye respond most strongly to red, green and blue wavelengths, together giving us all the variety of colors.

Yeah, so what does physics say about why the sky is blue?

Tyndall effect

Made the first steps towards a correct explanation of the color of the sky John Tyndall in 1859. He discovered a curious effect: if you pass light through a transparent liquid in which small particles are suspended, blue light will be scattered by these particles more than red light.

This can be easily demonstrated. Take a glass of water and stir a few drops of milk, a little flour or soap in it until the water in the glass becomes cloudy. Then shine a flashlight through the glass. You will see that the light inside the glass has become bluish. Or rather, the light that entered your eyes from the glass turned bluish, that is, it was rejected and scattered in the solution!

But the most interesting thing is that the light coming out of the glass, having lost part of its blue component, will no longer be white, but yellowish! If you take a sufficiently wide container, then the light, having scattered many times along the way, will finally lose its blue component and will come out of the container no longer yellow, but red.

The Tyndall effect concerns the scattering of light in turbid liquids. Particles in such a liquid must have a special surface structure - grooves, gratings, pores, corners, the size of which is comparable to the wavelength of light.

Thanks to the Tyndall effect, beautiful blue sapphirinid crustaceans exist. These tiny animals, as if glowing from within, sometimes become completely invisible to the observer (light scattering goes into the ultraviolet region)…

The Tyndall effect is also responsible for blue eyes in humans!

Yes, yes, blue eyes are not created by blue pigment at all - it simply isn’t there - but melanin, which diffuses the light accordingly!

A few years later, the Tyndall effect was studied in detail by Lord Rayleigh. Since then, the scattering of light by very small particles has come to be called Rayleigh scattering. Rayleigh showed that the amount of scattered light is inversely proportional to the fourth power of the wavelength for sufficiently small particles. It follows that blue light on such particles is scattered more than red light, approximately 10 times: (700 nm/400 nm) 4 = 10

Dust or molecules?

All this is wonderful, but our sky is filled with air, not liquid, and there are no pieces of soap or milk floating in the sky... What kind of particles scatter light in the air? Tyndall and Rayleigh believed that the blue color of the sky must be due to small particles of dust and droplets of water vapor, which are suspended in the atmosphere exactly as particles of milk are suspended in water.

This is a misconception, although even today some people say that the color of the sky is determined by steam and dust. If this were so, then the color of the sky would change much more depending on humidity or fog than it actually does. Therefore, scientists assumed (correctly!) that oxygen and nitrogen molecules were sufficient to explain the scattering. It is the air itself, or rather, its molecules scatter light!

Blue sky and clouds on it. Air scatters light in accordance with Rayleigh scattering, and larger cloud particles in accordance with Mie scattering. Photo: Andrei Azanfirei/Flickr.com

The question was finally resolved by Albert Einstein in 1911, who calculated a detailed formula for the scattering of light depending on molecules and further experiments brilliantly confirmed his calculations. They say that Einstein was even able to use his calculations as an additional test of Avogadro's number!

Why is the sky blue and not purple?

By the way, if blue light scatters 10 times more than red light, then even shorter violet waves should scatter more than blue ones! The question arises: why doesn't the sky look purple?

Firstly, the emission spectrum of light from the sun is not the same at all wavelengths - the maximum energy in the solar spectrum occurs in green light. Secondly, short-wave violet light is actively absorbed in the upper layers of the atmosphere (as is ultraviolet!), so less violet than blue reaches the Earth's surface.

Finally, the third reason is our eyes less sensitive to violet light than blue light.

Sensitivity curves for three types of cones in the human eye.

We have three types of color receptors, or cones, in the retina of our eyes. They are called red, blue and green because they react most strongly to light at these wavelengths. But in fact, they are capable of capturing light of other wavelengths, covering the entire spectrum.

When we look at the sky, red cones respond to small amounts of scattered red light, but also - less strongly - to orange and yellow wavelengths. Green cones respond to yellow and more strongly scattered green and green-blue waves. Finally, blue cones are stimulated by colors at blue wavelengths, which are highly scattered. If there were no blue and violet in the spectrum, the sky would appear blue with a slight greenish tint. But the most strongly scattered wavelengths of blue and violet lightly stimulate the red cones as well, so these colors appear blue with an added red tint. The overall effect is that When we look at the sky, red and green cones are stimulated approximately equally, and blue ones are stimulated more strongly. This combination ultimately creates a blue or blue sky.

Beautiful sunsets

What could be more beautiful than quiet sunsets on the seashore or in the steppe? When the air is clear and clear, the sunset will be yellow, just like a flashlight beam crossing a glass of soap: some of the blue light will scatter and the overall color of the Sun will shift to the red end of the spectrum.

Sunsets can be extremely varied in color depending on the state of the atmosphere. Photo: Alex Derr

It’s another matter if the air is polluted with small particles - fumes, dust, smog. In this case, the sunset will be orange and even red. Sunsets over the sea can also appear orange due to salt particles suspended in the air, which can create the Tyndall effect. The sky around the sun is seen reddened, as well as the light coming directly from the sun. This is because all light scatters relatively well at small angles, but then blue light is more likely to be scattered twice or more over larger distances, leaving yellow, red and orange.

Clouds, blue moon and blue haze

Clouds and dust haze appear white because they are made up of particles with longer wavelengths of light. Such particles will scatter all wavelengths equally (Mie scattering).

But sometimes there can be much smaller particles in the air. Some mountainous areas are famous for their blue haze. Terpene aerosols from vegetation react with ozone in the atmosphere to form fine particles about 200 nm in diameter that are excellent at scattering blue light.

Blue haze over the Bay of Kotor in Montenegro. Photo: Rocher/Flickr.com

A forest fire or volcanic eruption can sometimes fill the atmosphere with small particles with a diameter of 500–800 nm, which is the appropriate size for scattering red light. This is the opposite of the normal Tyndall effect and can cause the Moon to appear blue as red light from the Moon is scattered by these particles. Real blue Moon- a very rare occurrence!

Why is the sky of Mars red?

Now we have reached Mars, the sky on which, judging by the photographs of rovers and automatic descent vehicles, is red, sandy yellow, grayish blue... What is it really like?

According to physics, the Martian sky should be blue. It and there is blue, but only when the atmosphere on the Red Planet is calm. However, winds are known to blow frequently on Mars. Despite the fact that the planet’s atmosphere is extremely rarefied, the winds are capable of raising millions of tons of sand and dust and causing real sandstorms. Some storms can obscure almost the entire surface of Mars!

After such storms, particles of iron-rich dust remain suspended in the air for a long time. The color of this dust is red (this is rust), and accordingly the sky on Mars turns yellowish-orange.

Reflection nebulae

Finally, let's look far into space, to where stars are now being born.

The nebula complex of Ophiuchi. Photo: Jim Misti/Steve Mazlin/Robert Gendler

Here is a whole complex of cosmic gas and dust clouds located on the border of the constellations Ophiuchus and Scorpio. Please note: some of the clouds glow brightly with a reddish glow, the other part, on the contrary, absorbs light and resembles black holes. Finally, the third part has a bluish color.

All three types of clouds are composed mainly of hydrogen with a small admixture of dust and molecules. Why do they look different? It's all about their temperature. Heated by the light of the stars immersed in them, the clouds themselves begin to glow. The red glow is hydrogen radiation. Very cold clouds, on the contrary, absorb light and are therefore opaque to us. Finally, clouds that are cold but located close to bright stars appear bluish. They reflect the light of stars, scattering it in the same way as the Earth's atmosphere!

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