Update cheatsheet.en.md

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

@@ -42,3 +42,23 @@ Plane mirror, concave and convex spherical mirror
 ![](plane-concave-convex-mirrors.png)
 Fig. 1. a) plane    b) concave    c) convex   mirrors
 
+#### Are plane and spherical mirrors stigmatic?
+
+##### Perfect stigmatism of the plane mirror
+
+* A plane mirror is **perfectly stigmatic**.
+* Object and image are symmetrical on both side of the surface of the plane mirror.
+* A real object gives a virtual image.<br> A virtual object gives a real image.
+
+##### Non stigmatism of the spherical mirror
+
+* In each point of the spherical mirror, the law of reflection applies.
+* 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.)
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