Mirages english

1332 mots 6 pages
INTRO :

Everyone have already seen a scene like this before, or like this... We call this phenomenon mirage and that's what we will explain to you today.
First, the main thing to know about mirages is that they are not an optical illusion like many people can think... In fact, everyone is able to see and photograph mirages.Mirages are deformed pictures of a real object. Different sorts of mirages exist, due to the refraction and the reflexion of the light.

REFRACTION AND REFLEXION:

In a homogeneous and transparent environment, the light follows a straight direction. But all middle doesn't have the same refractive index (it's a index which can give an idea from the light speed). More the index is high, more the light go slowly.
* If the light passes from a middle with a low refractive index in a middle with a high refractive index, the direction of the light is transformed : It goes away from the boundary layer between the two environments.

* Otherwise, the light can also crosses from a middle with high index in a middle with less index. In this case, the direction of light gets closer of the boundary layer. (The pencil appears to be broken, due to refraction of light as it emerges into the air.)

* If the ray of light passes from a middle to another with an incidence angle more important that the critical angle, there is a total reflexion : The light doesn't go through the boundary layer.

The refraction law tell us that the direction of the ray evolve in function of the refractive index. However, the index depends of the temperature and the pressure of the environment. So the direction of light depends of the temperature of the middle.

* In an environment no homogeneous, it's possible to have different refractive index according to the temperature or the composition of the middle... But the index doesn't change brutally thus the direction of the light either. That's why, if you put a laser ray in a mixed of water and salt, you'll see a

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