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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

@@ -108,7 +108,7 @@ When spherical refracting surfaces are used under the following conditions, name
 \- The *angles of incidence and refraction are small*<br>
 Then *the spherical refracting surfaces* can be considered *quasi-stigmatic*, and therefore they can be used to build optical images.
 
-Mathematically, when an angle $`\alpha`$ is small $`alpha < or \approx 10 ^\circ`$,  the following approximations can be made :<br>
+Mathematically, when an angle $`\alpha`$ is small $`\alpha < or \approx 10 ^\circ`$,  the following approximations can be made :<br>
 $`sin(\alpha) \approx  tan (\alpha) \approx \alpha`$, and $`cos(\alpha) \approx 1`$.
 
 *Geometrical optics limited to Gaussian conditions* is called *Gaussian optics* or *paraxial optics*.