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Why the sky is blue?. Children's Science Lab

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

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

Atmosphere

The atmosphere is the mixture of gases and other particles that surround the earth. Basically, the atmosphere consists of gaseous nitrogen (78%) and oxygen (21%). Argon gas and water (in the form of steam, droplets and ice crystals) are the next most abundant in the atmosphere, their concentrations do not exceed 0,93% and 0,001%, respectively. The Earth's atmosphere also contains small amounts of other gases, as well as the smallest 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 the place, weather, etc. The concentration of water in the atmosphere increases during torrential storms, as well as near the ocean. Volcanoes are capable of throwing huge amounts of ash high into the atmosphere. Technogenic pollution can also add various gases or dust and soot to the usual composition of the atmosphere.

Atmospheric density at low altitude near the Earth's surface is the highest, with increasing altitude it gradually decreases. There is no clear-cut boundary between the atmosphere and space.

Light waves

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

Electromagnetic waves propagate through airless space at a speed of 299,792 km/s. The speed of propagation of these waves is called the speed of light.

Light wave parameters

The radiation energy depends on the wavelength and its frequency. The wavelength is the distance between the two nearest peaks (or troughs) of a wave. Wave frequency is the number of wave oscillations 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 our eyes can see. The light emitted by the Sun or an incandescent lamp may appear white, but is actually a mixture of different colors. You can see the different colors of the visible spectrum of light by decomposing it into its components using a prism. This spectrum can also be observed in the sky in the form of a rainbow, which occurs due to the refraction of the light of the Sun in water droplets, acting as one giant prism.

Rainbow

The colors of the spectrum are mixed, continuously moving one into another. At one end of the spectrum is red or orange. These colors fade into yellow, green, blue, indigo and violet. Colors have different wavelengths, different frequencies, and different energies.

Propagation of light in the 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, light continues to propagate 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 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 collides with these large particles. Different colors of visible light are equally reflected by these particles. Reflected light appears white because it still contains the same colors it had before it was reflected.

Gas molecules are smaller than the wavelength of visible light. If a light wave collides with them, then the result of the collision can 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 emitted light 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 much more absorbed 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 part of the red, orange and yellow colors interact with the air.

However, many shorter wavelengths of light are absorbed by gas molecules. After absorption, the blue color is emitted in all directions. It is scattered all over the sky. Whichever way you look, some of this scattered blue light reaches the observer. Since blue light is visible everywhere overhead, the sky looks blue.

Scattered light of the blue sky

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

The sky is paler on the horizon

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 that scatters sunlight. On Earth, some of the short wavelengths of sunlight (blue and violet) are absorbed by scattering. The rest of the spectrum looks yellow.

Also, in space, the sky looks dark or black instead of blue. This is the result of the absence of an atmosphere, hence the light does not scatter in any way.

Black sky in space

Why is the sunset red?

As the sun goes down, the 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 to be different, ranging from orange to red. This is due to the fact that even more short-wavelength colors, blues and greens, are scattered. Only the long-wavelength components of the optical spectrum remain, which reach the observer's eyes.

At sunset the sun appears red

The sky around the setting sun can be painted in 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, and so 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 that 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 it would be extremely cold, the temperature range could range from -170 ° C at night to + 120 ° C during the day. The atmosphere also protects us from the harmful radiation of the Sun and space, absorbing and scattering 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 do not realize 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 by a factor of 10 for every 16 km 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 * 1019 molecules. At ground level, each molecule travels at about 1600 km/h, while colliding with other molecules at a rate of 5 billion times per second.

Air density also drops rapidly with altitude. At a height of 3 km, the air density decreases by 30%. People living near sea level experience temporary breathing problems when raised to this altitude. 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 pronounced boundaries, they change seasonally, and in addition, their parameters change at different latitudes.

Layers of the atmosphere

Separation of the atmosphere into layers depending on their molecular composition

• Homosphere
• Lower 100 km including Troposphere, Stratosphere and Mesopause.
• Makes up 99% of the mass of the atmosphere.
• Molecules are not separated by molecular weight.
• The composition is quite 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 distributed unevenly. When water vapor rises, it cools and condenses, then returning to the earth in the form of precipitation - snow and rain. The stratosphere itself is very dry.
• Ozone is another molecule whose distribution is uneven. (Read about the ozone layer in the stratosphere below.)

heterosphere

• Extends above the homosphere, includes the Thermosphere and the Exosphere.
• The separation of the molecules of this layer is based on their molecular weights. Heavier molecules such as nitrogen and oxygen concentrate at the bottom of the layer. The lighter ones, helium and hydrogen, dominate the upper part of the heterosphere.

Separation 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 the absorption of ultraviolet light
• The degree of ionization changes with altitude.
• Different layers reflect long and short radio waves. This allows radio signals propagating 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 about 70000 km, this height depends on the intensity of the solar wind. The magnetosphere protects us from the high-energy charged particles of the solar wind by keeping them in the Earth's magnetic field.

Separation of the atmosphere into layers depending on their temperatures

The height of the upper limit of the troposphere depends on the seasons and latitude. It extends from the earth's surface to a height of about 16 km at the equator, and to a height of 9 km at the North and South Poles.

The prefix "tropo" means change. The change in the parameters of the troposphere occurs due to weather conditions - for example, due to the movement of atmospheric fronts.

As the altitude increases, the temperature drops. Warm air rises, then cools and descends back to Earth. This process is called convection, it occurs as a result of the movement of air masses. The winds in this layer blow mainly vertically.

This layer contains more molecules than all the other layers combined.

Stratosphere - extends approximately from a height of 11 km to 50 km.

• It has a very thin layer of air.
• The prefix "strato" refers to layers or layering.
• The lower part of the Stratosphere is quite calm. Jets often fly in the lower Stratosphere to get around bad weather in the Troposphere.
• Strong winds known as high-altitude jet streams blow in the upper part of the Stratosphere. They blow horizontally at speeds 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/m3, it absorbs the sun's harmful ultraviolet rays very effectively, thus protecting life on earth. The ozone molecule is made up of three oxygen atoms. The oxygen molecules we breathe contain two oxygen atoms.
• The stratosphere is very cold, its temperature is about -55°C at the bottom and increases with height. The increase in temperature is due to the absorption of ultraviolet rays by oxygen and ozone.

Mesosphere - extends to altitudes of about 100 km.

• As the altitude increases, the temperature rises rapidly.
• Thermosphere - extends to altitudes of about 400 km.
• With increasing altitude, the temperature rises rapidly due to the absorption of very short wavelength ultraviolet radiation.
• Meteors, or "shooting stars", begin to burn up at altitudes of about 110-130 km above the Earth's surface.

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

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

Experiments with light

The first experiment - decomposition of light into a spectrum

For 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:

1. Fill a 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 essential for the correct conduct of the experiment.
2. Place a mirror under water 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.

Experiment with the decomposition of light into a spectrum

What's happening: The water and the mirror act like a prism, splitting the light into its color spectrum. This happens because the rays of light 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 strongly decelerated and change their direction more strongly. Red rays slow down and change their direction to a lesser extent. The light is split into its component colors and we can see the spectrum.

Modeling the sky in a glass jar

Materials needed for the experiment:

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

Experiment:

1. Fill a glass or jar 2/3 full with water, approximately 300-400 ml.
2. Add 0,5 to one tablespoon of milk to the water, shake the mixture.
3. Taking a glass and a flashlight, go to a dark room.
4. Hold a flashlight over a glass of water and point the beam of light 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. This will give the water a reddish tint. Place a flashlight under the glass and point the beam of light upwards while looking at the water from above. In this case, the reddish tint near the water will look more saturated.

Modeling the scattering of sunlight in a jar of water

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 the 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 flashlight appears red, as some of the blue rays have been removed due to light scattering.

Color mixing

You will need:

• pencil, scissors, white cardboard or a piece of drawing paper;
• colored pencils or felt-tip pens, a ruler;
• a mug or a large cup with a diameter at the top of 7-10 cm or a caliper.
• Paper cup.

How to conduct an experiment:

1. 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.
2. Color these seven sectors in the colors of the main spectrum - red, orange, yellow, green, blue, indigo and violet. Try to paint the disk as accurately and evenly as possible.
3. Make a hole in the middle of the disc and put the disc on the pencil.
4. 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 disc into it so that the pencil lead rests on the table, adjust the position of the disc on the pencil so that the disc does not touch the bottom of the cup and is above it at a height of 0,5..1,5 cm.
5. Quickly spin the pencil and look at the spinning disk, note its color. If necessary, adjust the disk and pencil so that they can rotate easily.

Color mixing experiment

Explanation of the phenomenon seen: the colors that paint the sectors on the disk are the main components of the colors of white light. When the disk spins fast enough, the colors seem to blend and the disk looks white. Try experimenting with other color combinations.

Publication: the-mostly.ru

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