If, in an interview, they ask us about the color of the sky, our answer is to say blue. If we remember the sunsets and sunrises, we can clarify a little more. But if they ask us why it is blue, probably a majority of the interviewees would not know how to answer.
A scientific explanation is needed, and science, in the planning of non-university education, is not considered part of the 21st century's culture. This prevents a good part of the population from enjoying science as one enjoys literature, art, or music. Let's see what an interviewee with a scientific culture answers: The blue sky's cause is the atmosphere. On a planet without an atmosphere, the sky is black. The earth receives white light from the sun. But white light contains all the colors ranging from blue-violet to red. Let's not forget that colors are also light.
Also, each color has a different energy, the largest being that of blue. White light can be broken down into its colors by a phenomenon called scattering. For example, by passing it through a glass prism. This is what happens to sunlight as it passes through the atmosphere. Rayleigh showed that the scattering of light is greater, including its energy.
Blue light is scattered more than other colors because it travels as shorter, smaller waves. Therefore, the color that manifests itself with the greatest intensity is blue. We can say that the blackness of space penetrates the clarity of the atmosphere. This has always moved painters. For example, for Yves Klein, there is a deep, nothing with a deep blue above it. For Van Gogh, blue makes the yellow of the sun shine with all its splendor.
If the atmosphere contains 'aerosols,' the scattering they produce reduces the intensity of the blue. The density of aerosols in the atmosphere decreases with height. Therefore, it is greater at the horizon because it is attached to the surface of the earth. Therefore the blue of the sky is clearer on the horizon. For the same reason, a prolonged drought gives clear skies while after a rain, the blue is more intense. This also explains that the blue is more intense in the mountain than in the valley.
What happens at sunset?
Sunlight passes through more of the atmosphere than in the morning. Effectively. Freehand draw two concentric circles, the first the earth and the second the atmosphere. Let's draw a point on the surface of the earth. At noon the sunlight is in the direction of the center of the earth, that is, perpendicular to its surface. At sunset, it comes tangent to the surface of the earth at that point. In the drawing, you can see that you have a much larger tour of the atmosphere. If we add to this the meteorological situation (more or less water vapor, clouds, etc.) and the density of aerosols, we can explain and understand the full range of colors offered by sunsets (red, yellow, purple, etc.).
In reality it might surprise you, but the sky is not blue, it is ... black! Half the time, the sky is dark, when it is night (especially in the countryside, and without a moon). And from space, the sky looks black too.
The sky is only blue because there is the Sun, and its light which is ... white, even if the Sun is often represented as yellow when we draw it.
The sky is only blue because there is an atmosphere around the Earth: the layer of gas that surrounds our planet. This atmosphere diffuses light. The light rays generally go in a straight line. However, milk or clouds are only white (or gray) if there is a source of light (at night, clouds are also black). Clouds and milk diffuse light, like the walls of a lighted room, and like the sky.
What is the color of the scattered light?
With a candle placed behind a finger, it appears illuminated, but in red. It is our body that diffuses preferentially in red. For diffusion by the atmosphere, diffusion is preferentially in blue, and less and less towards green, yellow and red (the order of the rainbow).
A funny consequence of this diffusion by the sky of the blue light emitted by the Sun, is that the Sun appears less blue than it really is, and more red and green than it is: the red-green mixture seems rather yellow to our eye.
When it sets, the almost horizontal sun crosses a longer distance, even losing its green: it appears red to us, in a sky with pretty colors. For the same reason (the length of the atmosphere crossed) but in contrast, at high altitudes, with a high sun, the sky is an unusually deep blue, because atmospheric diffusion is reduced.
Why then does the Moon or the stars not appear yellow to us? In fact, when it's on the horizon, we sometimes have a red moon. Especially our eye saturates colors differently, the intensity of the Sun being incomparable to that of the stars or the Moon.
The sky appears blue because it is the dominant color among those that originate from molecules in the atmosphere and propagate to our eyes. On the other hand, the Sun appears red when it sets and if there was no atmosphere surrounding the Earth, the sky would still be ... black. Explanations.
Our eyes are sensitive to only part of the radiation from the Sun. Of the full spectrum of radiation emitted by the sun, this visible component is called "light". It appears white to us; in fact, this white results from the superposition of all the colors going from blue to red, as the rainbows reveal. The "decomposition" of white light can also be done with a glass (or better still, crystal) prism. It is, for example, the cause of the iridescence of oil stains on a wet road. You can also get white by combining only three colors of the rainbow, taken one in the middle of the spectrum and the others at both ends. These three colors, called primary, are blue, green and red. If three light beams having these colors and the same intensity are superimposed on a screen, we get white. In general, the combination of two or three of these primary colors makes it possible to obtain all the others in the technique of "additive trichromy" used, for example, by television.
Light is a wave that travels through space much like waves on the ocean surface, or like sound in the air. These three phenomena present great analogies. They are characterized first of all by the amplitude of the disturbance ('the wave') which affects the environment in which it propagates: it is the height of the waves in the case of the swell, and it is the maximum value of the excess pressure caused by the passage of a sound wave in the air. Note that the intensity of a sound or that of a light wave varies as the square of this amplitude.
The various waves are also characterized by their wavelength. In the case of swells, it is the distance between two successive waves. In the case of a sound wave corresponding to a pure musical note, it is the distance which separates, at a given moment, two neighboring zones of excess pressure produced by this sound. In general, the disturbances observed in the presence of a single wave (a musical note, ripples on the surface of a pond caused by the throwing of a stone, etc.) are reproduced gradually. close. This periodicity has a spatial scale; it is this scale that the wavelength characterizes. In the case of light, the associated disturbance is electromagnetic in nature as JC Maxwell showed in the second half of the 19th century: the presence of light implies that of an electric field combined with a magnetic field, one and the other oscillating very quickly, in synchronism.
As we have seen, white light is the superposition of the different colors of the rainbow. What differentiates one color from another is their wavelength. Red light has a wavelength of about 0.7 microns (a ‘micron is equal to one millionth of a meter), while the wavelength of blue is about 0.45 microns. They are small lengths compared to common dimensions, but very large compared to atoms and molecules, which are on the order of ten-thousandth of a micron.
When the various waves that make up the white light from the Sun reach Earth, they first pass through the atmosphere, which is made up mainly of molecules of nitrogen and oxygen. The electric field associated with each of the waves acts on the constituents of these molecules. Comparatively massive, atomic nuclei (positively charged) hardly move under the attraction of the electric field while electrons (negatively charged), themselves, much lighter than atomic nuclei, move a little at the frequency of oscillations of the electric field. Faint, periodic, electronic processions ensue. Each of the molecules then behaves like a tiny antenna that emits a light wave of the same wavelength as the incident light. Since this antenna is small compared to the wavelength, the light is emitted in approximately all directions. In addition, and this is the essential point, the re-emitted light - we speak of scattered light - is the more intense the shorter the incident wavelength. It was Lord Rayleigh who first showed in 1871 that blue largely dominated in scattered radiation. Specifically, the intensity of this scattered radiation varies as the inverse of the fourth power of the wavelength, or a factor of nearly six in favor of blue compared to red. Thus the sky appears blue because it is the dominant color among those which come from the molecules of the atmosphere and which propagate up to our eyes.
On the other hand, the Sun appears red at sunset, because the light then crosses a much greater thickness of the atmosphere and has been 'stripped' of its blue component - mainly by water vapor and by aerosols which preferentially absorb short wavelength, that is, blue. Note also that if there was no atmosphere surrounding the Earth, the sky would still be black (with the stars in addition and a very luminous white ball, that of the Sun) as it is at night. This is how it appears from the Moon because it has no atmosphere.
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