解き方

n_{12} = \frac{n_2}{n_1} = \frac{\sin \theta_i}{\sin \theta_r}を使って、

n_{12} = \frac{\sin \frac{\pi}{4}} {\sin \frac{\pi}{6}} = \frac{\frac{1}{\sqrt {2}}}{\frac{1}{2}} = \sqrt{2} = 1.41

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