# Mirages english

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• Publié le : 1 avril 2011

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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 ofmirages 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 lowrefractive 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 oflight 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 pressureof 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 acurved line and not a straight line.
In the case of mirages, it's the difference of temperature which create changes of refractive index and consequently the light bent.

REFRACTION:

When the temperature of the ground is different from the temperature of the atmosphere, there is near the ground a thin couch of air in which the indication of refraction quickly varies, and rays of light becomecurves. The warmer air is divided up into micro-strata, creating a gradient of indication. It deforms the image of objects situated near the ground.
We can observe especially mirages on big desert areas or on roads and certain places where there isn't any vegetation. There are two types of mirages, the lower mirage and the superior mirage.
They are caused by opposite variations of temperaturecall vertical temperature gradient. They express this gradient by the number of degrees of temperature change per height.
The illusion part of the mirage generally come from our mind's interpretation of what the eye sees. When our eye sees a light ray coming from an object, our mind interprets the ray as a perfectly straight line from the object to our eye.
Lower mirage :

The lower mirageisn't reserved for the hot deserts. We usually observe it on roads in summer: the road warmed by the sun causes an important gradient of temperature (very warm air near the road), and the refractive index of the air is modify according to the height. This gradient is negative, because the warmer air is denser than the cool. A 3 degrees variation of temperature over a 1meter high is enough to seethis type of mirage. You have to be on a plane surface and over a long distance. The eye receives then the colour of the sky by a ray of light having got closer to the ground before being curved.
What we see is the image of the sky which is refracted by the hot air until come to a critical angle, and then the rays are reflected. It appears like water lying on the ground. The observer believes...