\textbf{173-} In the figure, a light ray refracts at point A in medium with refractive index $n_1$ to point B in medium with refractive index $n_2$, where $n_2$ is the refractive index of the second medium. If $AI = IB = L$ and the speed of light in the first medium is $V_1$, what is the travel time of light from A to B?
\textit{[Figure: Light ray traveling from point A through medium $n_1$, refracting at point I on surface N, then continuing through medium $n_2$ to point B]}
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(1) $\dfrac{L}{V_1}\!\left(1+\dfrac{n_2}{n_1}\right)$
(2) $\dfrac{L}{V_1}\!\left(1+\dfrac{n_1}{n_2}\right)$
(3) $\dfrac{2L}{V_1}\!\left(1-\dfrac{n_2}{n_1}\right)$
(4) $\dfrac{2L}{V_1}\!\left(1-\dfrac{n_1}{n_2}\right)$
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