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This question, which might seem naive at first glance, takes us straight into the great book of the laws that govern molecular physics. Because, yes, to understand why our sky appears blue, we must delve into the world of the infinitely small and explore how sunlight behaves when it passes through the Earth's atmosphere.
An invisible world: air particles
The air around us is not empty: it is filled with extraordinarily small particles, nitrogen and oxygen molecules. To give you an idea of their tiny size, imagine that you would have to line up 250,000 of them to be as thick as a single human hair. Each molecule is about 0.4 nanometers, a unit so small that it represents one billionth of a meter. It is precisely these specific physical characteristics that determine their interactions with light.
These molecules never remain still: they are constantly in motion, moving and rotating on themselves. When sunlight passes through the atmosphere, it encounters these countless particles on its path. The behavior of these particles in the face of photons of light obeys the fundamental laws of quantum mechanics.
Each photon, upon hitting a molecule, causes a brief perturbation of its energy levels. This excitation is instantly released in the form of a new photon emitted in a random direction. An interaction that we will detail in the following part and that the video of the YouTuber Antoine vs Physique below explains perfectly.
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Blue, Earth's favorite color
The revelation of the secret of the terrestrial azure goes back to the British scientist Lord Rayleigh who, in 1870, proposed an explanation for this atmospheric phenomenon. His work revealed a natural preference of our atmosphere for the shortest light waves. This particular affinity for blue is expressed in a precise mathematical relationship: blue radiation, vibrating at wavelengths of 400-450 nanometers, encounters a resonance sixteen times greater than that of the longer red waves (620-750 nanometers).
200% Deposit Bonus up to €3,000 180% First Deposit Bonus up to $20,000When a solar photon encounters an air molecule, then begins an waltz infinitesimal in three time. First step: the molecule captures the light energy, entering a state of ephemeral excitation. Second step: this absorbed energy causes an instantaneous reorganization of its electronic structure. Third step: the molecule releases this energy in the form of light, projecting a blue flash in a random direction. This sequence, repeating itself billions of times at each instant, transforms our atmosphere into an immense natural prism that preferentially selects and diffuses the blue hues of the solar spectrum.
The changing colors of the sky
The solar spectrum, observable in its natural decomposition during the formation of a rainbow, includes all visible radiation, from red to violet. This spectral sequence, memorized by the acronym ROY G. BIV (red, orange, yellow, green, blue, indigo, violet), undergoes selective scattering as it passes through the atmosphere. When this light passes through the atmosphere, the blue part is more easily scattered in all directions, which explains why we see the sky blue when looking in any direction.
The chromatic variations observed during sunrises and sunsets are explained by the increase in the optical path in the atmosphere. As the sun approaches the horizon, the light rays pass through a considerably greater atmospheric thickness. At these times of day, the blue light is so diffused that it gets lost along the way, letting through mainly the red and orange colors that then color the sky.
The absence of an atmosphere on the Moon perfectly illustrates the importance of this phenomenon: without molecules to diffuse the light, the lunar sky remains perpetually black. Other atmospheric components, such as carbon dioxide and methane, although fundamental to other atmospheric phenomena, contribute only marginally to these optical effects.
If one day your child asks you this famous question ” Mom/Dad, why is the sky blue ? “, then answer him more simply: ” The sun sends little colored lights everywhere. The blue ones bounce all over the sky and that's why we see it as blue ». An answer sufficiently effective and scientifically accurate to answer his question, without launching into a physics lesson.
- Sunlight interacts with tiny molecules in the air, which scatter short-wave colors, such as blue, more.
- The atmosphere acts as a natural filter, preferentially scattering blue light, while other colors dominate at sunrise and sunset.
- On the Moon, where there is no atmosphere, the sky remains black, demonstrating that the color of the Earth's sky is due to the scattering of light by air molecules.
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