We welcome the most inquisitive on the pages of our site! Today we will touch on a question that often worries inquisitive minds (especially children's), but not everyone finds an opportunity to figure out why is the sky blue because the air is actually transparent. Let's try to answer it briefly.
What does Wikipedia say?
If we don’t know something, then we can always find the answer in Wikipedia. So let's take a look there and see what this resource tells us.
As a matter of fact, here is a link to the relevant material.
Well said on Wikipedia! True, somehow it is not very clear. The only thing that can be sorted out is the fact that the sun's rays reach our atmosphere, something happens to them, and we see a blue sky. No, this will not work, let's try to understand in more detail and in a more understandable language why the sky is blue.
In fact, the reason for everything is such a thing as " light scattering»!
Light scattering
So, the sun emits rays that have White color. White, as you know, includes all the colors of the spectrum visible to us. Evidence of that - rainbow. It arises for the reason that sunlight, falling into water droplets, is refracted and breaks up into different colors. We also observe blue skies for a similar reason.
This is how a rainbow is formed You may be interested in:
The fact is that there are many gas molecules in the air, which scatter sunlight. Particles of light fly into different sides, so the blue color of the sky is visible to both earthlings and astronauts from the ISS in the form of a blue halo. But why is it blue, because there are at least seven colors in the spectrum, as they say: "Every hunter wants to know where the pheasant is sitting!".
Interesting fact! A person inhales about 20 kg of air per day. We get this volume by taking 22,000 breaths a day.
Why is the sky blue?
Each color has its own wavelength. In the following figure, you can see how this indicator varies.
Violet diffuses too much, and the colors from green to red, on the contrary, do not scatter very intensively. So it turns out, blue and blue particles are the golden mean. Violet, despite the fact that it scatters better than blue, we do not notice because of our perception: with the same brightness, blue is perceived by our eyes much better than its counterpart.
That's pretty much how it goes Here is a good video on this topic, which helped us understand this issue:
Let's summarize
- Sunlight, interacting with air molecules, is scattered into different colors.
- Of all the colors, blue is the best for scattering.
- It turns out that he actually captures the airspace.
Of course, the information provided is rather exaggerated and one should focus on many scientific facts and concepts, but it is more or less clear why the sky is blue.
It turns out that very few people know the answer to this seemingly "always hanging in the air question." Children often ask about this, but adults are not ready to give an explanation. Many people think that this question is one of a series of questions that we cannot answer at all, such as "where is the end of the universe." There are people who believe that this is the color of a mixture of nitrogen and oxygen when there are many of these gases and they are illuminated by the Sun. There are those who associate the color of the sky with the refraction of light in the layers of the atmosphere. Those who were excellent students at school will say that, they say, the air scatters the blue color more intensely than all other colors of the spectrum according to Rayleigh's law, often without understanding the essence of this scattering. By the way, the question of the color of the sky was solved by physicists only in the twentieth century. Therefore, we should not be especially ashamed.
And although this issue is not directly related to temperature, let's try to figure it out. We will not dig very deep into physics, but we will recall the basic provisions about light and air.
Sunlight is a mixture of radiations of all colors of the rainbow, i.e. electromagnetic waves with oscillation frequencies such that they can affect the human retina. Violet corresponds to a wavelength of 380 nm, red - 720 nm. The retina contains cones responsible for color perception. There are three types of cones: blue (for high frequencies), green (for mids), and red (for low frequencies). The sensitivity ranges of cones overlap, but the maximum falls on a certain color.
Air molecules in the normal state do not have a charge, they are neutral. However, they consist of charged particles - electrons and nuclei. Under influence electric field the nuclei are shifted in one direction, the electrons in the other, and a dipole is obtained with its own electromagnetic field. If the dipole enters an alternating electromagnetic field, then it begins to oscillate, that is, the positive and negative charges shift back and forth and the dipole itself begins to radiate an electromagnetic wave. In our case, the electromagnetic wave of sunlight causes air molecules to turn into radiant electromagnetic waves dipoles. Moreover, the directions of studying dipoles can be various. According to the law of conservation of energy, a light wave loses intensity in its original direction. This is the main mechanism of light scattering in air. It is more correct to speak not even about scattering, but about the glow of air molecules under the action of light. We look through the atmosphere and actually see the light from the sun and the light emitted by the molecules in our atmosphere. Why is it not white, but blue?
The fact is that the intensity of the dipole radiation is proportional to the fourth power of the radiation frequency. The most intensely emitted by dipoles are waves with maximum frequency and energy, corresponding to blue light. Waves of red light interact less with air molecules. Those. when passing through the atmosphere, it is as if white color is filtered along the spectrum. Air molecules emit mainly blue light, i.e. light that excites the blue and green cones of the retina much more strongly than the red cones.
John Tyndall was the first to take a step towards a correct explanation of the color of the sky in 1865. He discovered that when light rays pass through a medium in which small particles of impurities are suspended, the blue color is scattered more intensely than red. As a result, we see the coloration of the transmitted light in a blue tint. This can be observed when looking sideways at a beam of light passing through water slightly clouded with milk. If you look not from the side, but in the direction of the beam, then the light acquires a reddish tint, because. the blue component is scattered.
A few years later, the British scientist Lord Rayleigh studied this effect in more detail. He showed that the intensity of light scattering by very small particles is inversely proportional to the fourth power of the radiation wavelength. From this it followed that blue light scatters 10 times more intensely than red.
Tyndall and Rayleigh thought that the blueness of the sky was due to the presence of small particles of dust and water vapor in the atmosphere. Later, scientists realized that if this were true, then we would observe significantly more variations in the color of the sky with changes in humidity, nebula, and air pollution than we see now. The problem was solved by Einstein, who in 1911 derived a formula describing the scattering of light by molecules. The formula confirmed all previous experiments. It was proved that it is not dust and vapor, but air molecules that scatter light, since (as mentioned above) the electromagnetic field of light initiates electric dipole moments in molecules.
Why is the sky not purple, but blue? After all, purple waves are shorter than blue ones. The first reason is that the spectrum of solar radiation is not uniform. There is less purple. In addition, violet rays are scattered even in the most upper layers atmosphere. The second reason is that our cones are less sensitive to violet than to blue. The third reason is that the blue color irritates not only the blue cones in the retina, but also slightly red and green ones. Therefore, the color of the sky is not pale, but saturated blue, especially when the air is transparent.
The color of the sunset is also explained by the scattering of light by air molecules. Having traveled a long way from the Sun tangentially to the Earth, the beam loses all blue hues. Only yellow and red tones reach the eye. Near the sea, the sunset can even be orange, thanks to the salt particles in the air that are responsible for Tyndall's scattering.
Note that the composition of the atmosphere, i.e. the presence of nitrogen and oxygen, the color of the sky is practically independent. If the planet has a transparent atmosphere of sufficient thickness and density, illuminated by a luminary whose spectrum is white, like that of the Sun, then the sky there will be blue.
How, then, to explain that the pictures from spacecraft who have landed on Mars say that the sky is pink and red there? This is because the atmosphere of Mars is very thin and polluted with dust. Scattering of sunlight occurs not on molecules, but mainly on suspended impurities of dust. Many dust particles are larger than the wavelengths of light and are made up of iron oxide, which is red in color.
Now you know that the answer to the question "why is the sky blue" is not very simple. We understand, but what to say to the children? It is likely that our beautiful atmosphere consists of air that glows blue when the Sun warms it. Because blue is the strongest of all the colors in the rainbow.
In spite of scientific progress and free access to a variety of sources of information, a rare person can correctly answer the question why the sky is blue.
Why is the sky blue during the day?
White light - namely, it radiates from the Sun - consists of seven parts of the color spectrum: red, orange, yellow, green, blue, indigo and violet. The counting rhyme known from school - "Every Hunter Wants to Know Where the Pheasant Sits" - just determines the colors of this spectrum by the initial letters of each of the words. Each color has its own wavelength of light: the longest for red and the shortest for purple.
The sky (atmosphere) familiar to us consists of solid microparticles, tiny drops of water and gas molecules. Over time, there have been several misconceptions trying to explain why the sky is blue:
- the atmosphere, consisting of the smallest particles of water and molecules of various gases, passes the rays of the blue spectrum well and does not allow the rays of the red spectrum to touch the Earth;
- small solid particles - for example, dust - suspended in the air scatter blue and violet waves the least, and because of this they manage to reach the Earth's surface, unlike other colors of the spectrum.
These hypotheses were supported by many famous scientists, but the studies of the English physicist John Rayleigh showed that it is not solid particles that are the main cause of light scattering. It is the molecules of gases in the atmosphere that separate the light into color components. A white sunbeam, colliding with a gas particle in the sky, scatters (scatters) in different directions.
When colliding with a gas molecule, each of the seven color components of white light is scattered. In this case, light with longer wavelengths (the red component of the spectrum, which also includes orange and yellow) is scattered worse than light with short waves (the blue component of the spectrum). Because of this, after scattering, eight times more blue spectrum colors remain in the air than red ones.
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 purple, with the same brightness of both. It is these facts that determine the color scheme of the sky: the atmosphere is literally filled with blue-blue rays.
Why is the sunset red then?
However, the sky is not always blue. The question naturally arises: if we see blue skies all day long, why is the sunset red? Above, we found that red is the least scattered by gas molecules. During sunset, the Sun approaches the horizon and the sunbeam is directed to the Earth's surface not vertically, as during the day, but at an angle.
Therefore, the path it takes through the atmosphere is much Moreover that it passes during the day when the sun is high. Because of this, the blue-blue spectrum is absorbed in a thick layer of the atmosphere, not reaching the Earth. And longer light waves of the red-yellow spectrum reach the surface of the Earth, coloring the sky and clouds in the red and yellow colors characteristic of sunset.
Why are clouds white?
Let's touch on the topic of clouds. Why are there white clouds in the blue sky? First, let's remember how they are formed. Moist air, containing invisible steam, warms up near the surface of the earth, rises and expands due to the fact that the air pressure at the top is less. As it expands, the air cools. When a certain temperature is reached, water vapor condenses around atmospheric dust and other suspended solids, and as a result, tiny droplets of water are formed, the merger of which forms a cloud.
Despite their relatively small size, water particles are much larger than gas molecules. And if, meeting air molecules, the sun's rays are scattered, then when they meet water drops, the light is reflected from them. At the same time, the initially white sunbeam does not change its color and at the same time “paints” the cloud molecules white.
One of the hallmarks of a person is curiosity. Probably everyone, as a child, looked at the sky and wondered: “why is the sky blue?”. As it turns out, the answers to such seemingly simple questions require some knowledge in the field of physics, and therefore not every parent will be able to correctly explain to the child the reason for this phenomenon.
Consider this question with scientific point vision.
The wavelength range of electromagnetic radiation covers almost the entire spectrum of electromagnetic radiation, which includes radiation visible to humans. The image below shows the dependence of the intensity of solar radiation on the wavelength of this radiation.
Analyzing this image, one can note the fact that visible radiation is also represented by uneven intensity for radiation of different wavelengths. So a relatively small contribution to visible radiation makes the violet color, and the largest - blue and green colors.
Why the sky is blue?
First of all, we are led to this question by the fact that air is a colorless gas and should not emit blue light. It is obvious that the cause of such radiation is our star.
As you know, white light is actually a combination of radiation of all colors of the visible spectrum. Using a prism, you can explicitly decompose light into the entire range of colors. A similar effect occurs in the sky after rain and forms a rainbow. When sunlight enters the earth's atmosphere, it begins to scatter, i.e. radiation changes its direction. However, the peculiarity of the composition of air is such that when light enters it, radiation with a short wavelength is scattered more than long-wave radiation. Thus, taking into account the spectrum shown earlier, it can be seen that red and orange light will practically not change its trajectory, passing through the air, while violet and blue radiation will noticeably change their direction. For this reason, a kind of "wandering" short-wave light appears in the air, which is constantly scattered in this medium. As a result of the described phenomenon, it seems that short-wave radiation of the visible spectrum (violet, blue, blue) is emitted at every point in the sky.
The well-known fact of the perception of radiation is that the human eye can catch, see, radiation only if it directly hits the eye. Then, looking at the sky, you will most likely see the shades of that visible radiation, the wavelength of which is the smallest, since it is it that scatters best in the air.
Why don't you see a distinctly red color when you look at the Sun? Firstly, a person is unlikely to be able to carefully examine the Sun, since intense radiation can damage the visual organ. Secondly, despite the existence of such a phenomenon as the scattering of light in the air, nevertheless, most of the light emitted by the Sun reaches the surface of the Earth without being scattered. Therefore, all the colors of the visible spectrum of radiation are combined, forming light with a more pronounced white color.
Let us return to the light scattered by the air, the color of which, as we have already determined, should have the smallest wavelength. Of the visible radiation, violet has the shortest wavelength, followed by blue, and blue has a slightly longer wavelength. Taking into account the uneven intensity of the solar radiation, it becomes clear that the contribution of the violet color is negligible. Therefore, the largest contribution to the radiation scattered by the air is blue, followed by blue.
Why is the sunset red?
In the case when the Sun hides behind the horizon, we can observe the same long-wave radiation of red-orange color. In this case, the light from the Sun must travel a noticeably greater distance in the Earth's atmosphere before reaching the observer's eyes. In the place where the radiation of the Sun begins to interact with the atmosphere, blue and blue colors are most pronounced. However, with distance, shortwave radiation loses its intensity, as it is significantly scattered along the way. While longwave radiation does an excellent job of overcoming such long distances. This is why the Sun is red at sunset.
As mentioned earlier, although long-wave radiation is weakly scattered in air, there is still scattering. Therefore, being on the horizon, the Sun emits light, from which only the radiation of red-orange hues reaches the observer, which has some time to dissipate in the atmosphere, forming the previously mentioned "stray" light. The latter paints the sky in variegated shades of red and orange.
Why are clouds white?
Speaking of clouds, we know that they are made up of microscopic droplets of liquid that scatter visible light almost uniformly, regardless of the wavelength of the radiation. Then the scattered light, directed in all directions from the droplet, is scattered again on other droplets. In this case, the combination of radiation of all wavelengths is preserved, and the clouds "glow" (reflect) in white.
If the weather is cloudy, then solar radiation reaches the Earth's surface in an insignificant amount. In the case of large clouds, or a large number of them, some of the sunlight is absorbed, so the sky dims and takes on a gray color.
