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Why is the sky blue ?
Both in daylight and at night,
there are stars in the sky that emit light.
Among them is the Sun,
a much more closer star than the others
and therefore much more brighter.
There are also stars such as the Moon or the planets,
which do not emit light but are lit by the Sun.
And everywhere else,
the sky should be black
since there is no light.
Besides, this is how astronauts
in orbit around the Earth see it.
In daylight, they see a bright sunshine,
and a black sky full of stars.
But when we are on Earth, during the day,
we see the blue sky,
actually a blue light that comes from some place
where there is no source of light at all,
there is nothing.
Well this is not quite true.
There is the atmosphere, so there is air.
But the air is transparent:
the fact that we can see very well stars at night
is a proof.
This is true but not entirely,
because when light is sent into the air,
a part of this light is diffused,
that is to say it is deflected by the molecules.
Usually, light travels in a straight line.
But when a photon (a speck of light) meets a molecule,
it can be deflected in whatever direction.
And as there are a lot of photons coming from the Sun
and a lot of molecules in the Earth's atmosphere,
there are finally a whole lot of photons in every directions.
If the diffusion was very efficient,
stars would be blurred,
as if we were looking at them through the fog.
But only a small quantity of light is diffused,
therefore a powerful source of light is needed
so that the light diffused
can be discernible to the unaided eye.
This is the case for the Sun,
but not for the other stars,
which explains why the sky remains black at night.
Speaking of which,
as regards the efficiency of the diffusion,
the wavelength of the light intervenes as well:
the closer to the blue colour it is, the more it diffuses.
For example,
if the Sun is at its zenith,
only 4% of the red light is diffused,
but 9% of green will be diffused,
20% of blue and 30% of purple.
So if we look at the sky in another direction than the Sun,
we will mainly see purple and blue colours.
But as the eye is much more sensible
in the blue than in the purple,
we see the blue.
This is why the sky is blue.
If we look at the Sun,
the colours are distorted as well.
Because our Sun is utterly white,
as the astronauts in orbit around the Earth can witness.
But as the Sun light has lost some blue colour
while going through the atmosphere, it seems yellow.
If the atmosphere was thinner,
there would be less light diffused,
and the blue colour of the sky would be darker,
even almost black when the atmosphere becomes very thin.
This is why in the mountains,
the blue colour of the sky seems deeper.
And if the atmosphere was thicker,
we would lose even more blue, and even green;
thus the Sun would become red.
Actually, this is exactly what happens
during a sunrise or a sunset:
the air layer to cross is thicker,
and the Sun becomes orange at first, then red.
Production: Unisciel/ University of Lille 1
Conception/Production: Maxime Beaugeois, Damien Deltombe and Daniel Hennequin
Editing/Special effects: Perrine Lefrileux
Music: Sébastien Ride, « Thunder Chacha » (SR Music)
Presentation: Maxime and Nina Beaugeois
Graphic design/Credits animation: Michaël Mensier.