Update cheatsheet.en.md

01.curriculum/01.physics-chemistry-biology/02.Niv2/04.optics/03.optical-systems-vergence/02.optical-systems-vergence-overview/cheatsheet.en.md

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 * **Emerging ray** (or its extension) = *straight line between impact point and B'*.
 
+![To find the deviation of any incident ray](rayon-incident-quelconque-2.gif)
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 ##### Determining the incident ray corresponding to any emergent ray
 
 * **deflection of a ray by a thin optical element** : *local interaction the point of impact* (refraction or reflection) between  emergent ray and thin optical element $\Longrightarrow$ *independent of the distance of the image point*.