= Normal incidence
{title2=$\theta_i=0$}
At <normal incidence>, the incident <light ray> follows the surface <normal vector>. <Snell's law> gives $\theta_t=0$ for positive <refractive indices>, so the transmitted <light ray> does not bend. Nevertheless, its <phase velocity> and <wavelength> change if the <refractive index> changes. A single axial <light ray> therefore cannot measure apparent depth: neighboring <light rays> and their directions supply that information. A reflected component can still be present; no bending of the transmitted <light ray> does not imply complete transmission.
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