---
title: 'The 4 laws of the geometrical optics'
---

Law 1
: In an **homogeneous medium**, the *light rays travel in straight lines*
: Dans un **milieu homogène**, la *lumière se propage en ligne droite*

!!! EXAMPLE &nbsp;&nbsp;&nbsp;of homogeneous medium : vacuum, air, glass, water,  liquids, ...
!!! EXEMPLE &nbsp;&nbsp;&nbsp;de milieux homogènes : le vide, l'air, le verre, l'eau, les liquide, ...

Law 2
:Dans le vide, l'énergie de la lumière se propage en suivant les rayons lumineux à la vitesse $`c=300000 km/s = 3.10^8 m/s`$.
: In vaccum, the energy of the light travel along the light rays at the speed $`c=300000 km/s = 3.10^8 m/s`$.
!!! COMMENT the value of the speed in vaccuum, 3.10^8 m/s is the approximated value you have to remember. The speed of the light is not anymore measured, but fixed at the exact value of 
