| (1) $\dfrac{\sqrt{2}}{2}$ | (2) $1$ | (3) $\sqrt{2}$ | (4) $2$ |
\textbf{171.} According to the figure below, a light ray enters a transparent medium from air and refracts. This ray travels from point A to point B in how many nanoseconds?
$$\left(c = 3\times10^8\ \frac{\text{m}}{\text{s}}\right)$$
\textit{[Figure: A ray incident at $45^\circ$ from the normal entering a medium with $n_1=1$ (air) and $n_2=\sqrt{2}$, with vertical distance $15\sqrt{3}$ cm between A and B]}
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(1) $\dfrac{\sqrt{2}}{2}$ & (2) $1$ & (3) $\sqrt{2}$ & (4) $2$
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