diff --git a/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 b/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
index b5cf02432..47caa8d2d 100644
--- a/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
+++ b/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
@@ -128,8 +128,8 @@ We call **thin spherical refracting surface** a spherical refracting surface *us
 which defines $`\overline{SC}`$ : algebraic distance between vertex S and center C of curvature on optical axis.
 
 * 2 refractive index values :<br>
-\- **$`n-{inc} : refractive index of the medium of the incident light**.<br>
-\- **$`n-{eme} : refractive index of the medium of the emergent light**.
+\- **$`n-{inc}`$ : refractive index of the medium of the incident light**.<br>
+\- **$`n-{eme}`$ : refractive index of the medium of the emergent light**.
 
 * 1 arrow : indicates the *direction of light propagation*
 *