Translations:Ray equation for velocity linear with depth/4/en

${\begin{aligned}\mathrm {grad} \ {\textit {n}}&{\rm {=}}\left({\frac {\partial n}{\partial x}}{\rm {,\ }}{\frac {\partial n}{\partial y}}\right){\rm {=}}\left({\rm {0,\ }}{\frac {\partial n}{\partial y}}\right){\rm {=}}\left({\rm {0,\ }}-{\frac {a}{v^{\rm {2}}}}\right).\end{aligned}}$

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