# Translations:Reflection and refraction – book/23/en

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When the lower medium has a lower velocity than the upper medium that contains the incident ray, the ray is bent toward the normal. If the lower medium has a higher velocity, the ray is bent away from the normal. In this latter case, a critical value exists for the angle of incidence for which the angle of refraction is equal to ${\displaystyle 90^{\circ }}$. In this particular case, the refracted ray is parallel to and grazes along the refracting interface. The critical angle ${\displaystyle {\theta }_{c}}$ is given by the relation ${\displaystyle {\rm {\ sin\ }}{\theta }_{c}{\rm {=}}v_{\rm {1}}{\rm {/}}v_{\rm {2}}}$, because ${\displaystyle {\rm {\ sin\ }}{\theta }_{t}{\rm {=\ sin\ }}\left({\rm {9}}0^{\rm {o}}\right)}$ is equal to unity. If the angle of incidence is greater than the critical angle, no refracted ray exists. This implies that all energy in the incident wave is contained in the reflected wave, and the incident wave is said to be totally reflected.