We can use Snell's Law for this:
$n_1 \sin{\theta_1} = n_2 \sin{\theta_2}$
For total internal reflection, $\theta_2 = 90^\circ$, so $\sin{\theta_2} = 1$. We can rearrange the equation to find the critical angle $\theta_1$:
$\sin{\theta_1} =
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