The sky is blue physics. What color is the sky? Why is the sky blue in terms of physics? Why is the sky blue during the day

Have you ever wondered why the sky is blue? After all, the atmosphere consists of transparent air, and sunlight is white. How does it happen that during the day in the light of the Sun the sky becomes blue and opaque. Until 1899, this paradox was insoluble, but now science knows the answer.

Why is the sky blue?

The answer lies in the nature of light. White light consists of seven colors of the spectrum: red, orange, yellow, green, blue, blue and violet, each of which corresponds to a specific wavelength. The wavelengths of red light are the longest, orange a little shorter...violet are the shortest.

1. Sun
2. rays of light
3. The colors of the spectrum that make up the visible part of the radiation (light) of our Sun.
4. Earth

When passing through the dense earth's atmosphere, light begins to scatter, refracting on the smallest particles of gas, water vapor and dust. As you may have guessed, not all components of the spectrum scatter equally. So long red waves practically do not scatter to the sides, following the beam to the very ground. Blue short-wave light, on the contrary, is very well scattered to the sides, coloring the entire sky in blue-blue tones.

1. light waves
2. Earth's atmosphere
3. Refraction and scattering of the blue part of the spectrum

The shorter the wavelength of light, the more it scatters in the atmosphere, and vice versa. The number "3" in the figure marks the process of refraction of light on gas molecules, dust particles and water drops that fill the atmosphere.

Short answer: The blue part of the color spectrum of the Sun, due to the short wavelength, is better scattered in the earth's atmosphere compared to the other 6 colors of the spectrum.

Why is the sky NOT purple?

The violet part of the spectrum does indeed have a shorter wavelength than the blue part, and therefore it is better scattered in the atmosphere. However, our sky is not purple. Why? First, the Sun has an uneven spectrum - violet radiation is much less than blue. Secondly, human eyes are less sensitive to purple.

Why is the sunset red?

During sunrise and sunset, sunlight travels tangentially to the earth's surface - the distance traveled by the beam through the atmosphere increases significantly. All short-wavelength light is scattered to the sides long before it reaches the observer. Only long orange and red waves reach the ground, which are slightly scattered along the direct rays and color the local part of the sky.

Why the sky is blue?

“Dad, why is the sky blue and not, say, green or purple?”
Children when they start learning the world are very active in asking questions. Hundreds of questions a day about anything that comes under the spotlight. All you hear is "why-why". And dad (or mom) by no means can "fall face down" and lose authority, saying "I don't know." How can this even be, because he has been living for a very long time and knows such elementary things for sure from his childhood?
And dad, of course, knows why this sky is suddenly blue 😉, and if he suddenly forgot something, he carefully reads what is written below.

What color is sunlight?

To understand the color of the sky, and understand why it is, you first need to find out what color the sunlight has. This question seems elementary.
"Yellow" - the baby will tell you, but here he will have to be surprised for the first time.
"But it's not yellow!"
O_O - the baby will have approximately the same eyes (something is clearly wrong with dad).
“Come on, raise your head, dad! It's yellow! Why is it not? Very much yes!”
“But no!” Then dad makes an authoritative face and says:
“In fact, the color of the sun and its rays is white, and the fact that we see it yellow is because it becomes so after passing through the air.”

What is white made up of?

"What colors do you know?" the child's father asks.
"Green, yellow, red, white ..." - the kid begins to list.
“Clever! All those colors that you listed, except for white, are simple colors. But the white one is special! In nature, there is no just white, but it appears if you put together all the simple colors.
It's like in a game when you need to collect parts of an object. Here you take one part, the second, the third, etc., and when you collect everything - TADAM! You get the whole item! So is white - it consists of all colors, and if at least some shade is taken away from it, it will no longer be white. It's clear?"
"Yeah," the kid nods.

So what's up with the color of the sky? Why is it blue?

“All this is very interesting, but, in my opinion, you are moving away from the topic. How about the color of the sky? Why is it like this?
“I'm just getting there. I told you elementary things so that more complex ones could be explained on the fingers.
As for the sky, I must say this. Scientists have not yet found an absolutely exact answer, but there are two simple theories that explain why the shade of the sky is blue. I'll tell you both.

First theory:

In the air that surrounds the earth, a large number of particles fly - these are different gases, dust particles, water particles, etc. When a white ray from the sun (and, as you remember, it is not by itself, but all the colors together) enters the air, then colliding with particles of air and particles that fly in the air, it begins to crumble into the colors of which it consisted.
It turned out that not all of them are equally fast, some are very clumsy, they disperse in the air, colliding with some particles, while others, very fast, dodge collisions and reach the Earth.
Blue rays are clumsy, they most often fall on obstacles and scatter (scatter) in all directions, illuminating the air with blue light.

Second theory a little more complicated:

Scientists suggest that particles of the air that surrounds the Earth absorb the sun's rays. They, as it were, are charged from these rays, and then begin to radiate their own light in all directions.

Well, for example, like a door on a stove. Remember, I showed you how the door was black at first, and then warmed up, warmed up and began to glow red? Do you remember?
“Yeah, I remember. And why did you remember the stove? .
“Yes, because it is the same here. Particles of air receive energy from the sun's rays, and then begin to glow. Different gases glow differently. The fact that we see the sky blue, according to this theory, it is thanks to the gases that make up our air (oxygen and nitrogen) that they emit a blue color. But if instead of them there would be, for example, neon (there is such a gas), then the sky would glow red-orange, but we would not be able to enjoy this spectacle, because. couldn't breathe.
Therefore, I think that even if it remains blue, blue is also okay, right?
“I agree,” the kid nodded, and after a minute, when he saw the dog, he asked the following vital question: “Dad, and

There are millions of questions that, being children, we do not receive an answer, and when we grow up, we are simply embarrassed to ask. One of these unanswered questions: "why is the sky blue?" And everything would be fine, and without this knowledge you can live, but when a child begins to ask such tricky questions to his parents, they often become ashamed, and they begin to change the subject. Then the child grows up not knowing the answer, he has his own children and everything repeats again. Let's break this "vicious circle" and understand the reasons why the sky is blue. Consider the issue from all possible points of view.

The phenomenon of blue sky in terms of physics

Let's get straight to the point, the sky is blue because the earth's atmosphere scatters the light of the sun. All research conducted over the past 200-300 years comes down to this. Consider a few axioms that affect the blue sky phenomenon:

1. The white light of the sun is a combination of different color streams. White color "separately" does not exist. As everyone knows, there are only 7 colors (red, orange, yellow, green, blue, blue, purple), the rest of the colors are obtained only when they are combined. White color is obtained by combining all seven colors. It is worth considering that it is precisely the colors that we can distinguish with the eye that are meant.
2. The atmosphere is not empty, it consists of many gases: nitrogen (78%), oxygen (21%), carbon dioxide, water in its various states (steam, ice crystals). There is also a lot of dust around us, elements of various metals. All of them distort the white light of the sun.
3. The air that surrounds us and that we breathe is actually opaque. In any case, in large quantities. We do not live in a vacuum, after all.

From these three facts we will proceed further.

Story

Back in the 19th century, a scientist named John Tyndall conducted research that proved that we see the sky blue because of particles in the atmosphere. In his laboratory, he artificially created a fog with dust particles and directed a bright white beam at it - the color of the fog changed to bluish. 30 years later, in 1899, the physicist Rayleigh refuted the research of his predecessor and published evidence that the sky is blue because of air molecules and no dust in it. called this phenomenon « diffuse sky radiation You can read more about this on Wikipedia.

The sky looks blue because air scatters short wavelength light more than long wavelength light. Since blue light has a shorter wavelength, at the end of the visible spectrum, it scatters more in the atmosphere than red. (Source: Wikipedia)

What is light? Light is a stream of photons, some we can see with our eyes and some we can't. So, for example, we see the standard spectrum of colors, but the ultraviolet, which also emits the sun, does not. What color we see in the end depends on the "wavelength" of this stream. This wavelength determines what color you get.

So. We have determined that the sun sends us quanta with a wavelength that corresponds to white color, but how does it turn blue as it passes through the atmosphere? Let's take the example of a rainbow. Rainbow - is a direct example of the refraction of light and its division into a spectrum. You can create your own rainbow using a glass prism at home. The decomposition of color into a spectrum is called dispersion.

So, our sky functions as a prism. Most white light changes its wavelength as it passes through gas molecules in the atmosphere. As a result, photons “leaving” the molecules have a different color. This color can be either purple, red, or blue and blue.

Why do we see blue and not red?

What color we eventually see when light travels from the sun to the earth depends on which photons prevail. For example, when light passes through the atmosphere, the number of blue color quanta is 8 times more than red, and violet is 16 times! This is due to the very different wavelength, so violet and blue scatter strongly, and red and yellow scatter much worse. Based on this theory, the sky should be purple, but it is not. This is due to the fact that purple is much worse perceived by the human eye, unlike blue. That is why the sky is blue.

Video about why the sky is blue:

Why is the sky blue during the day and the sunset is red

Everything, again, is connected with the dispersion of color. The angle of incidence of solar white light becomes smaller, and the light passes through large quantity air molecules, the wavelength of light increases. This amount is enough to diffuse to red.

The answer to the question why the sky is blue for children

If a child asked you a question about the blue sky, you certainly will not tell him about dispersion, spectra and photons. It is enough to quote from the children's book "100 Children's Why" Tatiana Yatsenko:

Usually we draw the sun's rays in yellow. But in fact, the light of the sun is white and consists of seven colors. These are the colors of the rainbow: red, orange, yellow, green, blue, indigo, violet. Not all colors pass through the air, only blue, indigo and violet. They color the sky.

It would be enough. On our website, you can also download a presentation on the topic: “Why the sky is blue” at the link: It may come in handy in the classroom at school.

On a clear sunny day, the sky above us has a bright blue color. In the evening, at sunset, the sky takes on a deep red color with numerous shades that is pleasing to the eye. So why is the sky blue during the day? What makes a sunset red? How transparent air shimmers with blue and red hues in different time days?

I will present 2 answers here: the first one is more simplified for the general reader, the second one is more scientific and accurate. Choose for yourself which one you like.

1. Why is the sky blue and not green? Answer for dummies

Light from the sun or a lamp looks white, but white is actually a mixture of all 7 existing colors: red, orange, yellow, green, cyan, indigo, and violet (Figure 1). The sky (atmosphere) is filled with air. Air is a mixture of tiny gas molecules and small pieces of solid material such as dust. As sunlight travels through air, it collides with air particles. When a beam of light hits gas molecules, it can "bounce" in the other direction (scatter).

Some component colors of white light, such as red and orange, pass directly from the Sun into our eyes without scattering. But most blue rays "bounce" off air particles in all directions. Thus, the whole sky is literally pierced with blue rays. When you look up, some of this blue light reaches your eye and you see blue light from all over your head! Here, in fact, why the sky is blue!

Naturally, everything is simplified to the maximum, but below is a paragraph where the property of our beloved sky above our heads is more fundamentally described and the reasons that explain why the color of the sky is blue and not green after all!

2. Why is the sky blue? advanced answer

Let's take a closer look at the nature of light and color. Color, as everyone knows, is a property of light that our eyes and brain can perceive and define. Light from the sun is a large amount of white rays, which consist of all 7 colors of the rainbow. Light has the property of dispersion (Fig. 1). Everything is illuminated by the Sun, but some objects reflect rays of only one color, for example, blue, while other objects only reflect rays of yellow, etc. This is how a person defines colors. So, the Sun shines on the Earth with its white rays, but the atmosphere (a thick layer of air) envelops it, and when this white (consisting of all colors) ray passes through the atmosphere, it is the air that scatters (spreads) all 7 colored rays of the white sun ray, but with greater force, it is its blue-blue rays (in other words, the atmosphere literally begins to glow blue color). Other colors directly fall from the Sun into our eyes (Fig. 2).

Why is blue the color most diffused in the atmosphere? This a natural phenomenon, and it is described by the Rayleigh physical law. To put it more simply, there is a formula that Rayleigh deduced in 1871, and which determines how the scattering of light (a beam) depends on the color of this beam (that is, on such a property of the beam as its wavelength). And it just so happened that the sky-blue color has the shortest wavelength and, accordingly, the greatest scattering.

Why is the sky red during sunrise and sunset? At sunset or sunrise, the sun is low on the horizon, which causes the sun's rays to fall obliquely

yut to Earth. The beam length, of course, increases many times (Fig. 3), and therefore, at such a huge distance, almost the entire short-wavelength (blue-blue) part of the spectrum is scattered in the atmosphere and does not reach the Earth's surface. Only long waves, yellow-red, reach us. This is exactly the color the sky takes on during sunrise and sunset. That is why the sky, in addition to blue and blue, is also yellow and red!

And now, for a complete understanding of all of the above, a few words about what the atmosphere is like.

What is the atmosphere (vault of heaven)?

The atmosphere is a mixture of gas molecules and other materials that surround the Earth. Basically, the atmosphere consists of nitrogen gases (78%) and oxygen (21%). Gases and water (in the form of vapours, droplets and ice crystals) are the most common constituents of the atmosphere. There are also small amounts of other gases, as well as many fine particulate matter such as dust, soot, ash, salt from the oceans, etc. The composition of the atmosphere changes with geographical location, the weather and much more. Somewhere there may be more water in the air after a rainstorm or near the ocean, somewhere volcanoes erupt large amounts of dust particles high into the atmosphere.

The atmosphere is denser in its lower part, near the Earth. It gradually thins out with height. There is no sharp gap between atmosphere and space. That is why we see overflows of blue and blue in the sky, precisely because the atmosphere in the sky is different everywhere, has a different structure and properties.

Probably everyone at least once in their life has come across this simple question: Why is a clear, cloudless sky blue or blue? Obviously because of the air we breathe, because of the Earth's atmosphere! Probably our air is "blue" or something like that. It only seems transparent, and at great distances, planes, mountains, ships seem to be in a bluish haze ... Such reasoning does not remove 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.

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

But where did blue light come from? To begin with, the white light from the Sun is a mixture of all the colors of the rainbow, from purple to red. Stop, you say Is the sun's light white? Yes, . Second point: we are now talking about light and not about color. If we mix paints of different colors, then, of course, we 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 approximately equal amounts, we get white light. Isaac Newton was the first to demonstrate this, using a prism to separate different colors and form a spectrum.

Scientists have found that colored light is just light different lengths waves. The visible part of the spectrum ranges from red light at a wavelength of about 720 nm to violet at a wavelength of about 380 nm, between which are orange, yellow, green, blue and blue colors. Three various types The color receptors in the retina of the human eye respond most strongly to red, green, and blue wavelengths, adding up to giving us all the variety of colors.

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

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

This can be easily demonstrated. Take a glass of water and stir in it a few drops of milk, a little flour or soap, so that the water in the glass becomes cloudy. Then run a flashlight through the glass. You will see that the light inside the glass has become bluish. Rather, the light that got into your eyes from the glass became bluish, that is, it was deflected and scattered in the solution!

But the most interesting thing is that the light at the exit from the glass, having lost part of its blue component, will no longer be white, but yellowish! If we take a sufficiently wide container, then the light, having scattered many times on the road, will finally lose the blue component and 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, angles, the size of which is comparable to the wavelength of light.

Thanks to the Tyndall effect, beautiful blue sapphirinids exist. These tiny, as if glowing from within, animals 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 at all by blue pigment - it is simply not there - but melanin, which scatters light appropriately!

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 been 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 is scattered by such particles more than red light, by about 10 times: (700 nm/400 nm) 4 = 10

Dust or molecules?

All this is beautiful, 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 water vapor droplets, which are suspended in the atmosphere in the same way as particles of milk are suspended in water.

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

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

The question was finally solved by Albert Einstein in 1911, who calculated a detailed formula for the scattering of light as a function of molecules, and further experiments brilliantly confirmed his calculations. It is said that Einstein was even able to use his calculations as an additional verification of Avogadro's number!

Why is the sky blue and not purple?

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

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

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

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

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

When we look at the sky, red cones respond to a small amount 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, the blue cones are stimulated by colors at blue wavelengths, which scatter very strongly. 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 about the same, and blue ones are stimulated more strongly. This combination eventually forms 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 transparent, the sunset will be yellow, just like a flashlight beam crossing a glass of soap solution: 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

Another thing is if the air is polluted small particles- fumes, dust, smog. In this case, the sunset will be orange and even red. Sunsets over the sea can also be 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 scatter twice or more at longer distances, leaving yellow, red, and orange.

Clouds, blue moon and blue haze

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

But sometimes there can be much smaller particles in the air. Some mountainous areas are famous for their blue haze. Aerosols of terpenes from vegetation react with ozone in the atmosphere to form small particles with a diameter of about 200 nm, which are excellent at scattering blue light.

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

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

Why is the sky of Mars red?

So we got to Mars, the sky on which, judging by the pictures of rovers and automatic descent vehicles, is now red, now sandy-yellow, now 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, on Mars, as you know, winds often blow. Despite the fact that the atmosphere of the planet is extremely rarefied, the winds are capable of raising millions of tons of sand and dust, creating real sandstorms. Some storms can hide 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 is painted in a yellowish-orange color.

reflection nebulae

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

Complex of nebulae rho Ophiuchus. 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 of Ophiuchus and Scorpio. Pay attention: part of the clouds glows brightly with a reddish glow, the other part, on the contrary, absorbs light and resembles black dips. Finally, the third part has a bluish color.

All three types of clouds consist 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 the emission of hydrogen. Very cold clouds, on the other hand, 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 atmosphere of the Earth!

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