Update cheatsheet.en.md

01.curriculum/01.physics-chemistry-biology/02.Niv2/04.optics/04.use-of-basic-optical-elements/01.plane-refracting-surface/02.plane-refracting-surface-overview/cheatsheet.en.md

@@ -104,8 +104,8 @@ defined, and therefore the spherical refracting surface becomes *quasi-stigmatic
 #### Gauss conditions / paraxial approximation and quasi-stigmatism
 
 When spherical refracting surfaces are used under the following conditions, named **Gauss conditions** :<br>
-\- All *incident rays lie close to the optical axis*<br>
 \- The *angles of incidence and refraction are small*<br>
+(the rays are slightly inclined on the optical axis, and intercept the spherical surface in the vicinity of its vertex)<br>
 Then *the spherical refracting surfaces* can be considered *quasi-stigmatic*, and therefore they can be used to build optical images.
 
 Mathematically, when an angle $`\alpha`$ is small $`\alpha < or \approx 10 ^\circ`$,  the following approximations can be made :<br>