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Why does rainbows are curved?
Rainbows are caused by the sunlight reflection on a curtain of rain.
This is why we can see rainbows on rainy days,
when the sun makes an appearance.
The sunlight is white,
but it contains all the colours of the visible light,
red, purple including yellow, green and blue.
Furthermore, the light contains more than the visible colours,
namely ultraviolet and infra-red light.
The separation of these colours to produce the rainbow
occurs at every water drop of the curtain of rain:
the rays of the sun pass through the water drop
and are then reflected by the opposite wall of the drop
before it comes out forward.
So we might say that the water drop acts like a mirror,
but a mirror with particular properties.
When the light passes from air to another medium,
for instance water or glass,
it is deviated unless it enters perpendicularly at the separation surface.
This deviation explains us why,
when a half rod is soaked in the water, the rod seems to be broken.
But this deviation of the light is not similar for all the colours:
the purple and the blue are more deviated than yellow and red.
We cannot notice that by looking through a window because
we generally look with an angle approximately perpendicular to the window
and in particular because the window is not very thick.
However, if a glass prism is used,
its deviation is accentuated
and if we let enter the light almost in parallel with the face of a prism input,
we get a rainbow.
Let’s go back to our water drop:
once it enters in the drop, the sunlight is decomposed into its different colours.
Then, the light is reflected at the bottom of the drop,
and comes out with a different angle for every colour.
This angle can be accurately calculated,
the red one has an angle of 42.5°
and the purple one has an angle of 41.6°.
Every water drop acts as a mirror
but it’s a mirror that reflects every colour according to a different angle,
namely in a different location.
If the curtain of rain was replaced by a mirror,
we would see an image of the sun we know.
This image is moved and deformed with the curtain of rain:
we can see a series of images with different colours,
with blue colours shifted downward and the red colours upward.
As a result, the entire image is expanded
and is four times larger than the sun.
When we look at ourselves in a mirror,
we only see one image of ourselves.
But if there is a series of mirrors at the centre arranged in circle around us,
we can see as many images as there are mirrors,
and these images form a circle.
This is for the same reason that we can see a rainBOW:
at any place where water drop can reflect the sunlight toward us,
with a good angle, we see an image of the sun.
In fact, if we floated in the air
with the sun in a horizontal position behind us,
we would see a rainbow making a complete circle
with our shadow in its centre.
This is a thing we can experience by plane.
At the Earth’s surface,
when the sun is on the horizon behind us within the plains,
we only see one part of the rainbow
because the other part might be in the soil.
When the sun rises on the horizon,
the rainbow is less and less visible.
If the sun rises above 42 degrees,
for example, in Paris at 2 pm, at the end of March
and between 10h30 am and 5h30 pm at the end of June,
there are no rainbows...
The main rainbow may be associated to many others phenomena
that are more complex.
For instance, sometimes we see a second less powerful rainbow
outside the first one:
it comes from the rays of the sun
which are reflected twice inside the water drop.
It is possible to reproduce rainbows artificially
that are easier to observe and study
by covering a panel with glass beads that acts as water drops.
By lighting up this panel just with a halogen lamp,
the rainbow is reproduced.
Production: UNISCIEL/UNIVERSITE 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