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

@@ -96,7 +96,7 @@ $`sin(\alpha) \approx  tan (\alpha) \approx \alpha`$ (rad), et $`cos(\alpha) \ap
 $`\dfrac{1}{\overline{SA_{ima}}}+\dfrac{1}{\overline{SA_{obj}}}=\dfrac{2}{\overline{SC}}`$  (equ.1)
 
 * **Transverse magnification expression** :<br><br>
-$`\overline{M_T}=-\dfrac{\overline{SA_{ima}}}{\overline{SA_{obj}}}`$$&nbsp;&nbsp;(equ.2)
+$`\overline{M_T}=-\dfrac{\overline{SA_{ima}}}{\overline{SA_{obj}}}`$&nbsp;&nbsp;(equ.2)
 
 You know $`\overline{SA_{obj}}`$ , calculate $`\overline{SA_{ima}}`$ using (equ. 1) 
 then $`\overline{M_T}`$ with (equ.2), and deduce $`\overline{A_{ima}B_{ima}}`$.
@@ -109,6 +109,18 @@ then $`\overline{M_T}`$ with (equ.2), and deduce $`\overline{A_{ima}B_{ima}}`$.
 ! $`\overline{M_T}=+1`$.
 
 ! *USEFUL 2° :<br>
+! *You can find* the conjunction and the transverse magnification **equations for a plane mirror directly from 
+! those of the spherical mirror**, with the following assumptions :<br><br>
+! $`n_{eme}=-n_{inc}`$<br><br>
+! (to memorize : medium of incidence=medium of emergence, therefor same speed of light, but direction
+! of propagation reverses after reflection on the mirror)<br><br>
+!
+! are obtained by rewriting these two equations for a spherical refracting surface in the limit 
+! when $`|\overline{SC}|\longrightarrow\infty`$.
+! Then we get for a plane mirror :<br>
+! $`\overline{SA_{ima}}=\overline{SA_{obj}}`$ and $`\overline{M_T}=+1`$
+
+