Answer
$n=\sqrt 2$
Work Step by Step
We know that
$sin\theta=\frac{1}{n}sin(90^{\circ}-\phi)$
$\implies cos\phi=\sqrt{1-sin^2\phi}$
$\implies \frac{1}{n}=\sqrt{1-\frac{1}{n^2}}$ (because $sin\theta=\frac{1}{n}$)
$\implies n=\sqrt 2$
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