α_{c r} = arcsin (\frac{n_{C}}{n_{F}}) = 1, 488 r a d (= 85, 3^{\circ})

${α_{cr}=\arcsin⁡\left(\frac{n_C}{n_F} \right)=1,488\, \mathrm{rad}\, (= 85,3^\circ)}$