diff --git a/01.curriculum/01.physics-chemistry-biology/02.Niv2/04.optics/04.use-of-basic-optical-elements/02.mirror/02.new-course-overview/cheatsheet.en.md b/01.curriculum/01.physics-chemistry-biology/02.Niv2/04.optics/04.use-of-basic-optical-elements/02.mirror/02.new-course-overview/cheatsheet.en.md
index aa4dc948e..4d340162c 100644
--- a/01.curriculum/01.physics-chemistry-biology/02.Niv2/04.optics/04.use-of-basic-optical-elements/02.mirror/02.new-course-overview/cheatsheet.en.md
+++ b/01.curriculum/01.physics-chemistry-biology/02.Niv2/04.optics/04.use-of-basic-optical-elements/02.mirror/02.new-course-overview/cheatsheet.en.md
@@ -56,7 +56,37 @@ Fig. 1. a) plane b) concave c) convex mirrors
* A spherical mirror is not stigmatic: The rays (or their extensions)
* coming from an object point generally do not converge towards an image
* point (see Fig. 2.)
+*
+
+Fig. 2. Non stigmatism of a convexe mirror.
+
+Fig. 3. But when we limit the aperture of the mirror,
+
+
+Fig. 4 . and limit the conditions of use to small angles of incidence and
+refraction are small, then a point image can be defined : the mirror becomes
+quasi-stigmatic.
+
+* Spherical mirrors with a limited aperture (see Fig. 3.) and used so that
+angles of incense and emergence remain small (see Fig. 4.), become quasi-stigmatic.
+
+##### Gauss conditions / paraxial approximation and quasi-stigmatism
+
+* When spherical refracting surfaces are used under the following conditions, named **Gauss conditions** :
+\- The *angles of incidence and refraction are small*
+(the rays are slightly inclined on the optical axis, and intercept the spherical surface in the
+vicinity of its vertex),
+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 :
+$`sin(\alpha) \approx tan (\alpha) \approx \alpha`$ (rad), et $`cos(\alpha) \approx 1`$.
+
+* Geometrical optics limited to Gaussian conditions is called **Gaussian optical** or **paraxial optics**.
+
+#### The thin spherical mirror (paraxial optics)