diff --git a/01.curriculum/01.physics-chemistry-biology/04.Niv4/04.electromagnetism/02.electromagnetic-waves-vacuum/02.electromagnetic-waves-vacuum-main/textbook.fr.md b/01.curriculum/01.physics-chemistry-biology/04.Niv4/04.electromagnetism/02.electromagnetic-waves-vacuum/02.electromagnetic-waves-vacuum-main/textbook.fr.md
index a7c6ecc22..4111a1661 100644
--- a/01.curriculum/01.physics-chemistry-biology/04.Niv4/04.electromagnetism/02.electromagnetic-waves-vacuum/02.electromagnetic-waves-vacuum-main/textbook.fr.md
+++ b/01.curriculum/01.physics-chemistry-biology/04.Niv4/04.electromagnetism/02.electromagnetic-waves-vacuum/02.electromagnetic-waves-vacuum-main/textbook.fr.md
@@ -15,7 +15,11 @@ $`div \overrightarrow{B} = 0`$
 $`\overrightarrow{rot} \;\overrightarrow{B} = \mu_0\;\overrightarrow{j} +
 \mu_0 \epsilon_0 \;\dfrac{\partial \overrightarrow{j}}{\partial t}`$
 
-### Propagation du champ électromagnétique dans le vide
+### Rappel de l'équation d'onde d'un champ vectoriel
+
+$`\Delta \overrightarrow{X} - \dfrac{1}{v_{\phi}} \; \dfrac{\partial^2 \;\overrightarrow{X}}{\partial\; t^2}=0`$
+
+### Propagation du champ électromagnétique 
 
 $`\overrightarrow{rot}\,\overrightarrow{E}=-\dfrac{\partial \overrightarrow{B}}{\partial t}`$