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

@@ -208,9 +208,36 @@ propagation of the light (from the object towards the refracting surface)
 in Figures 3 and 4, and 5 and 6 below, and review NOTE 1.
 !
 
+#### Examples of graphical situations, with analytical results to train
 
+* with real objects
+
+[]()<br>
+Fig. 4.
+
+[]()<br>
+Fig. 5.
+
+[]()<br>
+Fig. 6.
+
+[]()<br>
+Fig. 7.
+
+* with virtual objects
+
+[]()<br>
+Fig. 8.
+
+[]()<br>
+Fig. 9.
+
+[]()<br>
+Fig. 10.
+
+[]()<br>
+Fig. 11.
 
-#### Non stigmatism of spherical refracting surfaces
 
 Ray tracing study of a **spherical refracting surface** :
 [Click here for geogebra animation](https://www.geogebra.org/material/iframe/id/gvkqgrpe)<br>