Translations:Snell's law/12/en

In the following derivation $\Delta t$ is a unit of travel time for a propagating wave. For a given $\Delta t$ the distance D₂ in the lower layer is less than the distance D₁ in the upper layer because $V_{2}<V_{1}$. The derivation hinges on recognition of the trigonometric relationships between the distance X along the refracting boundary and the distances D₁ and D₂.